chore - 1481 the kernel-side crates move into their rings

[dannymeijer]

Summary

Step 4a of the layout rewrite: the crates that are not toolchain support crates move into their rings by directory. incan_syntax, incan_semantics_core and incan_codegraph go to loaves/kernel/; rust_inspect and incan_semantics_stdlib go to loaves/compiler/ — the semantics packs are compiler implementation per layering.md, so they sit beside incan_provider rather than in the kernel; crates/third_party becomes loaves/third_party. Crate names stay what they are (step 5 renames incan_core → incan_lang, incan_semantics_core → incan_semantics, rust_inspect → incan_inspect); the directories carry the crate names until then, and each crate's README says so.

The five toolchain support crates stay under crates/ for now, deliberately: incan_core, incan_derive, incan_stdlib, incan_vocab and incan_web_macros are what the release archive ships under crates/, what generated projects and SDK components depend on by path, and what CI's INCAN_TOOLCHAIN_CRATES_DIR override points at as one directory. They move together with the toolchain path policy in 4b (the stdlib ring), as recorded on #1481.

What names the moved paths and follows them: every manifest path dependency (the root, the compiler-ring crates, the kernel crates' own ../incan_core/../incan_vocab); the compiler-suite target spelling for rust_inspect's lib in oven_model::compiler_suite_env; the third_party registry-patch directory policy in legacy_cargo (and its test); the effect-digest roots for the two semantics crates; the rustdoc gate's third_party skip rule; the semantic-string audit's roots and fixture entries for rust_inspect; the reporter label in one native-test test; and the in-repo docs that name the parser directory (AGENTS.md, the test skill and agent). The moved crates' rustdoc fills the width, like every crate moved before it.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/RFCs/*)

Area(s)

• Incan Language (syntax/semantics)
• Compiler (frontend/backend/codegen)
• Tooling (CLI/formatter/test runner)
• Editor integration (LSP/VS Code extension)
• Runtime / Core crates (stdlib/core/derive)
• Documentation

Key details

• User-facing behavior: none; the release archive's crates/ layout is untouched because the support crates did not move.
• Internals: six directory moves; Cargo.lock does not change (path dependencies record no paths). check_oven_ring still passes: no Oven crate depends on any moved crate.
• Risks: none beyond a path someone spells by hand; scripts/check_doc_paths.py (step 6) catches the docs-site ones.

Testing / verification

• cargo check -p incan in the six feature configurations; cargo clippy --all-targets default and --all-features with -D warnings; rustdoc gate last
• cargo test -p incan_syntax -p incan_semantics_core -p incan_codegraph -p rust_inspect -p incan_semantics_stdlib — 543 tests, the incan_syntax embedded-fragment tests reading the examples/pro consumers through one more ../
• cargo test --test cli_layering_guardrails --test vocab_guardrails --test layering_guard
• cargo test --test codegen_snapshot_tests — 266 unchanged
• make check-oven-ring
• root unit tests with the CLI built for cli::commands::inspect, cli::commands::oven, lsp
• one exact Oven root: make test-one TEST_ROOT=tests/cli_surface_tests.rs TEST_EXACT=a_cargo_manifest_beside_a_loaf_manifest_warns_without_stopping_the_build — bake + build, the compiler suite's target plan names rust_inspect's new path

Docs impact

• No docs changes needed beyond the in-repo path mentions updated here (the docs site is step 6)
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions

Part of #1481 (slice 5, #1478). Builds on #1577–#1581: the branch carries their commits, so the diff shows every step until they land and merge-commit syncs narrow it to this one.