dylint: ban raw `.fbuild` path literals outside fbuild-paths; ratchet to zero

[zackees]

Problem

fbuild-paths declares itself the single source of truth for all .fbuild paths (crates/fbuild-paths/src/lib.rs:3), but the .fbuild / .fbuild/build path segments are hardcoded as raw string literals in 69 files across the workspace — including production code:

• fbuild-cli/src/cli/{ide,debug,symbols_cmd,ide_debug,clang_tools,args,dispatch,lib_select}.rs
• fbuild-build-engine/src/{linker.rs,build_info.rs,symbol_analyzer,framework_libs.rs}
• fbuild-daemon/src/{handlers/libraries.rs,handlers/emulator,models.rs}
• fbuild-deploy/src/{lpc.rs,wlink.rs,wchisp.rs,probe_rs.rs,lpc_debugger_reflash.rs}
• fbuild-core/src/{response_file.rs,path.rs,lib.rs}
• plus ~25 test files hand-rolling fixture layouts like dir.join(".fbuild/build/uno/release/...")

Raw literals drift from the real layout rules (env-segment auto-collapse, FBUILD_BUILD_DIR override, PIO .build/pio layouts — see agents/docs/path-conventions.md) and silently break compile_cwd_from_output / BuildLayout consumers when the layout evolves.

Proposal

1. New dylint lint dylints/ban_raw_fbuild_path (pattern after the existing dylints/ban_raw_subprocess crate): flag string literals containing .fbuild outside crates/fbuild-paths/src/** (the sanctioned definition site).
2. Export canonical primitives from fbuild-paths: FBUILD_DIR_NAME / BUILD_DIR_NAME consts plus the existing project_fbuild_dir() / BuildLayout::resolve() so call sites have a mechanical replacement.
3. Ratchet: land the lint with a baseline allowlist of the current 69 files; each file must be sanitized (route through BuildLayout / the new consts) and removed from the allowlist; the allowlist may only shrink. CI fails on any new raw-literal site.
4. Test fixtures should build layouts via fbuild_paths::BuildLayout::new(project, env, profile).resolve() instead of hand-rolled join(".fbuild/build/...") chains.

Acceptance

• dylints/ban_raw_fbuild_path lint crate exists, wired into the dylint CI gate alongside ban_raw_subprocess
• Baseline allowlist checked in; count recorded in this issue
• Allowlist reaches zero (or only fbuild-paths definition sites remain)

[zackees]

Acceptance items 1 and 2 landed in #1353 (merged as 49ec69c).

Item 1 — lint crate wired into the Dylint gate. dylints/ban_raw_fbuild_path flags any string literal whose contents include .fbuild. It catches both real-world shapes — Path::join(".fbuild") segments and format!(".fbuild/build/{}/release/...") templates — because rustc lowers format_args! pieces to HIR Lit nodes, so the late pass sees them. Discovered via [workspace.metadata.dylint] libraries = [{ path = "dylints/*" }]; registered in the root exclude list, ci/hooks/crate_guard.py, and dylints/README.md.

Item 2 — baseline recorded: 47 files. Plus one permanent exemption, crates/fbuild-paths/src/lib.rs, the definition site itself.

The baseline came from a comment-stripping, raw-string- and escape-aware Rust string-literal scanner rather than a line grep, then was trued up against cargo-dylint dylint --lib ban_raw_fbuild_path -- --workspace --all-targets, which now exits 0 with zero violations. Two notes for whoever picks up the ratchet:

• crates/fbuild-cli/src/cli/args.rs is on the list because clap's #[derive(Subcommand)] stringifies /// doc text into help strings; the literal's call site is the #[derive] item, so the whole file is exempt rather than one doc line.
• assert!(cond) synthesizes "assertion failed: <stringified cond>" with no source span at all. Those are dropped by a dummy-span guard — a diagnostic nobody can locate is one nobody can fix.

Item 3 — ratchet to zero — remains open, which is why this issue stays open. The list is shrink-only and that is now enforced, not just documented: ci/check_fbuild_path_baseline.py diffs src/allowlist.txt against origin/main in the Dylint job and fails on any added entry. Removals always pass.

Also exported for the sanitization work: fbuild_paths::FBUILD_DIR_NAME and fbuild_paths::BUILD_DIR_NAME, alongside the existing get_project_fbuild_dir, get_project_build_root, and BuildLayout::resolve. Per-file recipe is in dylints/ban_raw_fbuild_path/README.md → Rollout: sanitize, delete the line, bump the crate version to bust the Dylint .so cache.