Land default-namespace fixes on main

[olavoasantos]

Problem

PR #678 was merged after its parent PR #677, but GitHub left its base set to the already-merged normalize-dom-namespaces branch. The merge therefore advanced that branch instead of main.

The #677 normalization work is present in main, but #678’s isDefaultNamespace() correction, tests, and changeset are not.

Impact

The reviewed namespace-context fix is marked merged without being part of the release branch. The remaining dependent PRs also cannot safely continue until this layer is reachable from main.

Change

Replay the exact reviewed #678 patch on current main:

• normalize the requested namespace before resolving receiver context;
• locate the default namespace iteratively from elements, documents, attributes, and ancestors;
• preserve fragment, document-type, and detached-attribute behavior;
• retain the 10,000-level regression coverage that prevents recursive stack overflow.

The stable patch ID is unchanged from #678 (c379a3b9dd0e45940251ae608c2c7f4cdbf8e136). This PR does not add new behavior beyond the already-reviewed layer.

Tests

Local validation on current main passes:

• lint;
• type-check and build;
• 44 test files / 515 tests;
• bundle-size checks.

Fresh GitHub CI is still required on this replacement head.

Ordering

This replaces the incorrectly targeted merge from #678. Land it before retargeting #679 and the remaining #677-dependent stack roots to main.