test: move docs inventory snapshot next to its test

[dasiths]

Problem

SnippetCompilationTests.Documentation_FrozenSurface saves its generated docs inventory (a SHA-256 hash for every inventoried Markdown file and code block) inside a dated plan document: .agent/plans/2026-09-11-aauth-v11-spec-migration/docs-surface-map.md. As a result:

• every docs change, even a typo fix, has to edit a plan folder from the September draft-11 migration;
• the failure message (Documentation inventory is stale…) doesn't say which file to update or how. docs: fix Mermaid sequence diagrams broken by semicolons #52 hit this and needed docs: regenerate docs surface inventory after Mermaid fixes #53 to fix it.

Change

• New: tests/AAuth.Tests/Api/DocumentationInventory.snapshot.md holds the generated inventory, with a header explaining how to regenerate it. It is three levels below the repo root, like the old file, so the generated ../../../ links still resolve. tests/ isn't inventoried, so the snapshot doesn't hash itself.
• Test: now reads and writes the snapshot, and the stale failure names the file and the AAUTH_UPDATE_DOCS_INVENTORY=1 step.
• Plan doc: the hand-written draft-11 analysis stays in docs-surface-map.md. Only the generated appendix was removed, and a dated update note points to the new location.

The test's behavior is unchanged: same inventory, same checks, same opt-in regeneration.

Validation

• After regenerating, the snapshot's generated section is byte-identical to the appendix it replaces.
• dotnet test tests/AAuth.Tests --filter "FullyQualifiedName~SnippetCompilationTests|FullyQualifiedName~DocumentationLinkTests": 108 passed.
• Negative check: appending a blank line to docs/glossary.md makes the test fail with the new message (Documentation inventory snapshot tests/AAuth.Tests/Api/DocumentationInventory.snapshot.md is stale…).