roadmap: replace the inherited JP base with a KR-first v2 pipeline

[junghan0611]

Goal

Build the next major GLG-Mono pipeline as a KR-first font, removing the inherited PlemolJP / IBM Plex Sans JP build structure as the architectural base.

This is not a cleanup patch to the current v1 pipeline. It is a parallel v2 rewrite with explicit acceptance gates.

Why

GLG-Mono inherited its build architecture from yuru7/PlemolJP. That was a practical starting point, but it leaves a Korean font structurally dependent on a Japanese base:

• IBM Plex Sans JP is opened as the base face;
• IBM Plex Sans KR is merged onto it;
• FONT_NAME = PlemolJP remains an internal compatibility invariant;
• Japanese comments, naming, selection rules and FontForge round-trips encode upstream assumptions;
• multiple local fixes now compensate for behavior that a KR-first pipeline should own directly.

Upstream's latest commit is 55e5011, dated 2025-06-28. This project should not plan around future upstream work. We will maintain and evolve the Korean font ourselves, with agent-assisted engineering and reproducible local verification.

Strategic decision

• KR is the primary CJK design source and architectural base.
• IBM Plex Mono remains the Latin programming face unless a separate decision changes it.
• Han/Kana coverage must become an explicit product decision. If IBM Plex Sans JP remains as a supplementary glyph source, it must be an isolated coverage input—not the base that controls the entire build.
• The current v1 pipeline remains frozen as a reproducible legacy reference until v2 passes its gates.
• Do not perform a big-bang rewrite in fontforge_script.py on main.

Work plan

1. Freeze and inventory v1

• Record canonical hashes and metadata for the release source faces.
• Inventory every source font and explain which codepoints/features it contributes.
• Inventory FontForge round-trips, width transforms, alt-unicode handling, glyph overrides, NF paths and legal records.
• Classify inherited files as required, replaceable or dead.
• Preserve the current desktop and eight-file webfont verifiers as regression oracles.

2. Specify the v2 composition contract

• Define source precedence for Latin, Hangul/Jamo, symbols, Han, Kana, box drawing and PUA.
• Decide whether Kana and the current Han repertoire remain in the desktop font.
• Define 1:2 and 3:5 metrics without relying on PlemolJP internals.
• Define Regular/Bold/Italic/BoldItalic behavior, including true Latin italic and intentional CJK oblique policy.
• Define GSUB/GPOS, combining-mark, zero-advance and vertical-metric contracts.
• Define legal provenance and name-table contents for every retained source.

3. Build KR-first in parallel

• Create a small, staged pipeline with explicit intermediate artifacts.
• Open IBM Plex Sans KR as the primary CJK face.
• Merge IBM Plex Mono and any supplementary coverage through declared policy tables rather than implicit selection order.
• Replace internal PlemolJP naming and compatibility assumptions.
• Keep generated artifacts out of Git and make clean builds deterministic.
• Avoid introducing new opaque FontForge save/reopen cycles; every unavoidable round-trip must have a verifier.

4. Acceptance gates

• Complete modern Hangul syllables, Jamo and compatibility Jamo.
• Correct Korean bearings and fixed-width alignment in all release faces.
• Latin source precedence, true italics and zero-advance glyphs preserved.
• Declared Han/Kana/symbol coverage exactly matches the v2 specification.
• Decomposed outlines, advances and LSBs checked against the selected source of truth.
• GSUB/GPOS shaping canaries pass for Latin combining text, NFD Korean, Jamo, Han/Kana and ligatures.
• Vertical metrics, hinting, BASE, legal name records and copyrights pass.
• Nerd Fonts, if retained, pass the same width and bearing gates.
• The eight-file {core,jp} web delivery contract remains reproducible or is replaced only by an explicit new decision.
• Two clean builds produce byte-identical artifacts.
• Linux terminal, browser and macOS/VS Code canaries pass before v2 replaces v1.

5. Retire the inherited structure

• Remove dead PlemolJP/PlemolKR scripts, stale docs and duplicate build paths.
• Remove FONT_NAME = PlemolJP and internal Plemol naming.
• Update repository history/provenance documentation without erasing upstream credit.
• Keep only source assets and notices required by the final v2 composition.
• Archive the v1 build instructions and migration notes.

Exit criteria

This issue is complete when:

1. GLG-Mono builds from a KR-first pipeline without PlemolJP as a structural/runtime dependency;
2. any JP source is an explicit supplementary coverage module;
3. the declared desktop and webfont quality gates pass on clean deterministic builds;
4. v1 legacy code is removed or archived only after v2 is accepted.

Non-goals

• Rewriting the active v1 webfont work while it is still being stabilized.
• Removing upstream copyright or provenance records.
• Silently dropping Han/Kana coverage to make the rewrite easier.
• Assuming an upstream maintainer will solve this architecture for us.