fix(evolution): look up COSE HeaderMap labels by structural equality (#482)

[Kshot3000]

Summary

Fixes #482: HeaderMap stored headers in a JS Map keyed by Label objects, but Label equality is structural while Map.get/Map.set compare by reference. Three consequences, all reproduced with failing tests on main first:

• (a) the public header() accessor called this.headers.get(label) with a freshly built Label, so it always returned undefined;
• (b) HeaderMap's [Equal.symbol] looked keys up in the other map by reference, so two structurally equal HeaderMaps never compared equal;
• (c) setAlgorithmId followed by setHeader(labelFromInt(1n), ...) inserted two distinct Label objects for logical key 1, leaving duplicate entries in the map — the exact sequence EdDSA25519Key.build() performs.

Fix

In packages/evolution/src/cose/Header.ts, following the pattern the working accessors (algorithmId, keyId, …) already use:

• a getLabel helper looks entries up by iterating with Equal.equals — header() and [Equal.symbol] now use it;
• a setLabel helper removes any structurally equal key before inserting, and every mutator (setAlgorithmId, setCriticality, setKeyId, setContentType, setInitVector, setPartialInitVector, setHeader) goes through it, so setting an already-present label overwrites instead of duplicating.

The CBOR encoding is unchanged: encode already re-keyed by primitive label.value, where the later entry won, and that is the entry the overwrite now keeps.

One scope note: value comparison inside [Equal.symbol] still uses Equal.equals, which compares Uint8Array values by reference — that is the pre-existing value semantics across this codebase and is not changed here.

Tests

New file packages/evolution/test/HeaderMap.LabelEquality.test.ts (8 tests), each verified RED on main (run one at a time, since this repo's vitest bails a file after its first failure) and GREEN with the fix:

• header() returns the value stored under an equal text label (the regression test from COSE HeaderMap: header() and equality use Map reference equality; build() creates duplicate label entries #482) and under an equal int label; returns undefined for a label never set and for a text label "7" vs int label 7;
• two HeaderMaps built from distinct but equal labels compare equal in both directions; maps with different values under an equal label do not;
• setHeader on an existing logical key leaves one entry holding the new value;
• the setAlgorithmId → setHeader(label 1) sequence leaves a single label-1 entry, and EdDSA25519Key.build() produces no duplicate label-1 entries (header(label 1) is the key type, header(label -1) the curve).

Full suite: 1686 passed / 75 skipped / 0 failed, test files 102 passed / 1 skipped of 103 — exactly main's 1678 baseline + the 8 new tests. tsc --noEmit, eslint and prettier are clean on both touched files, and a patch changeset is included.

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