Use Compass AST cache v1 namespace

[forhappy]

What changed

• replace the historical Graphify-derived cache/ast/v0.9.21/e6 namespace with Compass-owned cache/ast/v1/e6
• remove the public AST cache version override so production Compass has one current namespace
• hard-delete stale v* AST namespaces, including v0.9.21, when the cache opens
• document that v1 is AST compatibility while e6 independently identifies the binary encoding

Why

Compass retained 0.9.21 as a hard-coded compatibility-era value after the native extraction pipeline moved into Compass. That made the cache path look tied to the Graphify package version even though it was neither dynamic nor Compass-owned.

This is an intentional hard cutover: the first build after upgrading is cold, with no fallback or migration from the old namespace. Semantic and Program IR caches are unchanged.

Validation

• cargo test -p compass-files --all-targets --locked — 26 tests passed
• cargo clippy -p compass-files --all-targets --locked -- -D warnings
• cargo fmt -p compass-files -- --check
• git diff --check
• graphify update .