Fix dead nav links and add migrations index page

The VitePress nav menu had been carrying four dead targets and two
semantically wrong ones since the sidebar was populated. The `Guide`
entry (and its Chinese mirror `指南`) pointed at the site root `/`,
which renders the home page's hero+features layout rather than a
guide. The `Migrations` and `Changelog` entries (and their Chinese
mirrors) pointed at `/migrations/` and `/changelog`, which both
returned HTTP 404 — neither path had an underlying page. The nav
was the only place the four 404 URLs lived; nothing else linked at
them, which is why the strict dead-link checker and the prior
site-wide verification swept past them. They surfaced through a
manual menu-by-menu inspection of the deployed nav.

The `Guide` entry now points at `/guides/getting-started`, matching
the first entry in the Guides sidebar group. The home-page hero
"Get started" CTA continues to point at `/quickstart` separately,
so the 60-second and 15-minute entry paths stay distinct (hero =
quickstart; nav menu = guides region entry). The Chinese mirror is
updated symmetrically to `/zh/guides/getting-started`.

The `Migrations` entry retains its `/migrations/` target but the
URL now resolves through a new overview page,
`docs/migrations/index.md`. The page lists the six per-release
migration guides (v0.1→v1.0 through v1.4→v1.5) with one-line
descriptions of what each covers, a recommended reading order for
multi-version upgrades, an explanation of why this section is
English-only, and cross-links to the CHANGELOG, Getting started,
and API reference. It carries the `EN_ONLY_PREFIXES` hreflang-skip
classification from the prior commit (zh-CN hreflang omitted on
this and the six migration files alike), so search engines see a
single English-only section.

The Chinese `迁移指南` entry now cross-links to the same
`/migrations/` overview because migration guides are English-only by
policy — there is no `/zh/migrations/` namespace. The Chinese
sidebar already cross-links the per-release migration pages to
their English URLs the same way; the nav entry just makes that
explicit at the top-level navigation surface.

The `Changelog` entry (and its Chinese mirror) now points at the
canonical CHANGELOG file on GitHub at
`https://github.com/zhh2001/jp4/blob/main/CHANGELOG.md`. The
absolute-URL approach matches the same pattern already used inside
the migration guides for their CHANGELOG cross-references — the
CHANGELOG is a single canonical source and GitHub renders it
correctly, with no maintenance burden from mirroring it as a
VitePress page that would have to track every release.

Post-fix verification: VitePress builds in 6.5 seconds with zero
dead-link warnings; the sitemap grows from 62 to 63 entries (the new
migrations index lands as a sitemap URL); the live nav menu now
resolves all eight of its links (four EN + four CN); doc-lint
reports zero failures with the same 19 file-level skips as before;
the new index page carries unique title/description/keywords
frontmatter and renders through the per-page keywords meta the
prior commit wired up.

No Java source or test touched; the Maven build, test suite, and
published artefact surface are unchanged. SemVer-irrelevant.

Author: zhh2001

docs/.vitepress/config.ts

@@ -176,10 +176,10 @@ export default defineConfig({
       description: DESCRIPTION_EN,
       themeConfig: {
         nav: [
-          { text: 'Guide',         link: '/' },
+          { text: 'Guide',         link: '/guides/getting-started' },
           { text: 'API reference', link: '/api/' },
           { text: 'Migrations',    link: '/migrations/' },
-          { text: 'Changelog',     link: '/changelog' },
+          { text: 'Changelog',     link: 'https://github.com/zhh2001/jp4/blob/main/CHANGELOG.md' },
         ],
         sidebar: [
           {
@@ -283,10 +283,10 @@ export default defineConfig({
       description: DESCRIPTION_ZH,
       themeConfig: {
         nav: [
-          { text: '指南',     link: '/zh/' },
+          { text: '指南',     link: '/zh/guides/getting-started' },
           { text: 'API 参考', link: '/zh/api/' },
-          { text: '迁移指南', link: '/zh/migrations/' },
-          { text: '更新日志', link: '/zh/changelog' },
+          { text: '迁移指南', link: '/migrations/' },
+          { text: '更新日志', link: 'https://github.com/zhh2001/jp4/blob/main/CHANGELOG.md' },
         ],
         sidebar: [
           {

docs/migrations/index.md

@@ -0,0 +1,38 @@
+---
+title: Migration guides
+description: Per-release migration guides for jp4 — v0.1 → v1.0 through v1.4 → v1.5. Each guide documents the API surface changes between two adjacent versions, with before/after code snippets for callers updating across the boundary and the v1.x roadmap snapshot captured at that release.
+keywords: [jp4, migration, version upgrade, v0.1, v1.0, v1.1, v1.2, v1.3, v1.4, v1.5, release notes]
+---
+
+# Migration guides
+
+Per-release migration guides for upgrading a jp4 controller across one or more minor-version boundaries. Each guide documents the API surface changes between two adjacent versions: which methods were added, which behaviour tightened, what (if anything) requires a code change at the call site, and the v1.x roadmap snapshot captured at that release.
+
+The guides are written from the *callers* perspective — what does a controller already running on version X need to change to upgrade to version Y? The answer is "nothing" for every v1.0 → v1.5 transition because the project's SemVer commitment is binding: minor versions add capabilities without removing or breaking the existing surface. Each guide cites the commit hash where each change landed; `git show <hash>` resolves the full diff.
+
+## All guides
+
+| Guide | Released | What it covers |
+|---|---|---|
+| [v0.1 → v1.0](/migrations/migration-0.1-to-1.0) | 2026-05-08 | Additions (paramInt / paramLong / metadataLong / symmetric fromBytes factories), behaviour tightening (MastershipStatus.toString format, UncheckedIOException for fromFile, null-rejection on accessors, actionable width-overflow messages), documentation polish (thread-safety contracts, null contract, production readiness). The v1.0 cut. |
+| [v1.0 → v1.1](/migrations/migration-1.0-to-1.1) | 2026-05-09 | ReadQuery.where for client-side filtering, Connector.preserveRoleOnReconnect for primary-mandatory HA, Mac.ZERO constant. |
+| [v1.1 → v1.2](/migrations/migration-1.1-to-1.2) | 2026-05-11 | Packet-ingestion control surface — DropEvent record + Reason enum, P4Switch.onPacketDropped listener, Connector.packetInFilter pre-fan-out filter. |
+| [v1.2 → v1.3](/migrations/migration-1.2-to-1.3) | 2026-05-14 | Stream-message dispatch family completes — DigestEvent + P4Switch.onDigest + DigestConfig + P4Switch.enableDigest + IdleTimeoutEvent + P4Switch.onIdleTimeout + TableEntry.idleTimeoutNs. |
+| [v1.3 → v1.4](/migrations/migration-1.3-to-1.4) | 2026-05-15 | Per-entity-type read surface completes — counter / meter / register / action-profile member / action-profile group reads with the shared query-builder shape. BMv2 register-read UNIMPLEMENTED limitation surfaced. |
+| [v1.4 → v1.5](/migrations/migration-1.4-to-1.5) | 2026-05-15 | Packet replication engine reads — multicast group + clone session via name-less typed query builders. Shared Replica / BackupReplica records (BackupReplica is the first P4Runtime-1.5.0-spec-level type to land in jp4). |
+
+## Reading order
+
+If you're upgrading several versions at once (e.g. v1.1 → v1.5), read each intermediate guide in order — each one is scoped to a single adjacent transition, and concatenating their advice covers the full distance.
+
+If you're starting fresh on v1.5 and just want to know what's new since you last looked, the [v1.4 → v1.5 guide](/migrations/migration-1.4-to-1.5) is the most recent.
+
+## Why this section is English-only
+
+Migration guides document a fixed past API delta — they describe what changed in a specific release and do not get retranslated each release. The guides are linked from both the English and Chinese sidebars; the underlying content is English.
+
+## Related
+
+- [CHANGELOG](https://github.com/zhh2001/jp4/blob/main/CHANGELOG.md) — release-note form of the same information, plus the v1.x roadmap.
+- [Getting started](/guides/getting-started) — entry guide for users on the current version (v1.5).
+- [API reference](/api/) — categorised navigation of the v1.5 public surface plus the full Javadoc.