COSE HeaderMap: header() and equality use Map reference equality; build() creates duplicate label entries

## Summary
HeaderMap stores headers in a JS `Map` keyed by `Label` objects, but `Label` uses structural equality while `Map.get`/`Map.set` use reference equality. As a result:
- (a) the public `header()` accessor calls `this.headers.get(label)` with a freshly built `Label`, so it always returns `undefined`;
- (b) `HeaderMap` `[Equal.symbol]` looks up keys from `this` in `that`'s map by reference, so two structurally-equal HeaderMaps never compare equal;
- (c) `setAlgorithmId` followed by `setHeader(labelFromInt(1n), ...)` inserts two distinct `Label` objects for logical key 1, leaving two entries in the map.

No functional or security impact today: the security paths (`verifyData`, `algorithmId`, `keyId`) iterate the map manually with `Equal.equals` and are unaffected, `header()` is unused, and the duplicate label-1 entries collapse to a single correct entry on CBOR encode (encode re-keys by primitive value). Latent correctness / cleanup.

## Affected
packages/evolution/src/cose/Header.ts
- header() (L260-262): `this.headers.get(label)` uses reference equality -> always undefined
- [Equal.symbol] (L43-51): `that.headers.get(key)` uses reference equality -> HeaderMaps never equal

packages/evolution/src/cose/Key.ts
- EdDSA25519Key.build() (L248-252): `setAlgorithmId(1n)` then `setHeader(labelFromInt(1n), ...)` create two entries at logical key 1

## Fix
Stop keying a JS `Map` by objects with custom equality. Either intern `Label` instances so structurally-equal labels share a reference, or replace the map lookups (`header()`, equality, `setHeader` overwrite) with `Equal.equals`-based iteration (as the working accessors already do). Making `setHeader` overwrite an existing logical key also removes the duplicate label-1 entries in `build()`.

## Regression test
- given: a HeaderMap built with `setHeader(labelFromText("address"), bytes)`
- before fix: `hm.header(labelFromText("address"))` returns undefined
- after fix: returns the bytes
Must FAIL on main today and PASS after the fix.

## Reference
Reported informally (HeaderMap reference-equality). No live security impact; cleanup for a broken public accessor and a fragile build path.

Start with packages/evolution/src/cose/Header.ts, especially header() and [Equal.symbol], then inspect packages/evolution/src/cose/Key.ts around EdDSA25519Key.build(). Add the described regression test for a text label, and verify that header() returns the bytes, structurally equal HeaderMaps compare correctly, and setHeader does not leave duplicate logical labels.

Written by the indexing model from the issue text.