Support block-form nesting and unquoted assertions; compile doc examples in CI

[rokoss21]

Follow-up to #9. I compiled every ```facet block in the documentation of both repositories — 43 blocks — and two more documented constructs turned out never to have been implemented.

Block-form nesting (§5.1–§5.2)

Facet bodies accepted only inline literals, so a key followed by an indented map or list failed with F003. That is how the specification prints its own @context minimal schema:

@context
  budget: 32000
  defaults:
    priority: 500
    min: 0

…and how docs/02-tutorial.md writes @policy rule lists. Neither compiled.

block_body now parses a nested block when a key carries no inline value, and a list item may itself be a block map (- id: ... with further pairs indented beneath). Verified equivalence: block and inline spellings produce the same AST, and an equivalent policy yields an identical policy_hash (sha256:7397fb8c… both ways) while document_hash differs as it should.

Unquoted @test assertions (§15.1)

The spec writes assertions as bare expressions, but the parser demanded a quoted string, so §15.1's own example failed:

@test "basic"
  assert:
    - canonical contains "hello"

assert: items are now read as raw lines. Quoted items still work, so examples/spec/08_test_suite.facet and any existing document are unaffected. A false assertion still fails the test — this is not a blanket "accept everything".

Keeping docs conformant

scripts/check_doc_examples.py builds every fenced facet block with the reference compiler, wired into CI as Doc Examples Compile. Blocks that are meant not to compile — negative examples, syntax fragments, imports of paths that do not exist — live in scripts/doc_examples_allowlist.json with a reason, keyed by content hash so entries survive a block moving within a file. An allowlisted block that starts compiling is reported as stale, so the list cannot rot. Verified both directions: an intentionally wrong entry is caught.

This is the guard for the class of problem that started this whole thread: documentation that teaches syntax the compiler rejects is what newcomers and LLM assistants copy first.

Also fixed

• docs/02-tutorial.md: the @policy example omitted name, which §16.2.2 requires for tool_* rules — the documented policy raised F452. (Found only after the parser stopped failing first.)
• CHANGELOG.md carried a second, duplicated copy of the v2.0 changelog.

Verification

• 484 workspace tests pass, 8 new in tests/spec_conformance_regressions.rs, including equality of block vs inline ASTs, equality of quoted vs unquoted assertions, and that a dangling key: is still an error
• smoke + spec matrix suites green; doc-example check green over 43 blocks (13 allowlisted)
• cargo fmt --all -- --check and cargo clippy --all-targets --all-features -- -D warnings clean

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