Type-design: AllocationFailed.unit is redundant with AllocationContext

[r6e]

Source

PR #144 R1 type-design review (MEDIUM finding, deferred to follow-up).

Problem

PR #133 added unit: BoundsUnit to AllocationFailed (parallel to BoundsExceeded::unit). PR #134 typified context: AllocationContext. After both, the unit is structurally derivable from the context — verified by inventorying every call site:

AllocationContext variant	Unit
EntryPayloadBytes	Bytes
V10MainIndexBytes	Bytes
V10EncodedEntriesBytes	Bytes
V10FdiBytes	Bytes
InlineCompressionBlocks	Items
EncodedCompressionBlocks	Items
FlatIndexEntries	Items
DedupTracker	Items
ByPathLookup	Items
V10NonEncodedEntries	Items
V10IndexEntries	Items

The variant naming convention IS the discriminator: *Bytes ↔ Bytes, everything else ↔ Items. So AllocationFailed::unit is the type-system equivalent of carrying around a copy of self.id in a struct field — at every construction site the caller has to repeat data the type already knows, and at every reconstruction site there's an opportunity for the two to disagree silently.

No test pins the implicit unit == context.unit() invariant. A future call site that passes unit: BoundsUnit::Items with context: V10MainIndexBytes would silently render "items for v10+ index" instead of "bytes for v10+ index" — wrong but accepted.

BoundsExceeded::unit does NOT have this problem because WireField's units are mixed (UncompressedSize is bytes; BlockCount is items; CompressionMethod doesn't even have a sensible unit). Just AllocationContext.

Fix

Two acceptable resolutions:

Recommended (single source of truth):

1. Add pub fn unit(&self) -> BoundsUnit method to AllocationContext with the mapping above.
2. Drop the unit: BoundsUnit field from AllocationFailed.
3. Update Display arm to call context.unit() instead of reading the field.
4. Migrate all 11 production call sites + tests to drop the redundant arg.

This is mechanical and produces compile errors at every drift site — no silent failures possible.

Acceptable (paper over the gap):

1. Keep unit as-is.
2. Add AllocationContext::unit() -> BoundsUnit method.
3. Add a unit test that walks every variant and asserts context.unit() == matching unit field passed at the corresponding production site (or just that the method returns the documented value).
4. Document in AllocationFailed's unit field doc that it MUST equal context.unit().

The current "explicit-and-redundant" choice is the worst-of-both-worlds: every call site has to remember the right pairing AND nothing catches it if they don't.

Files

• crates/paksmith-core/src/error.rs (variant + Display + new method)
• All 11 AllocationFailed construction sites (5 production files + tests)