fbuild bloat: rename 'symbols' subcommand + drop flag dependencies (Phase 2 of #434) #438

Background

Phase 2 of #434 (the fbuild bloat meta). Depends on #428 (Phase 1 —
build_info toolchain paths), which lands via #436.

fbuild symbols works but the name is wrong (symbolize in industry
usage means address→symbol resolution for crash dumps). cargo bloat
and Google's bloaty are the de-facto names for per-symbol size
analysis. Align the CLI vocabulary.

What to do

1. Rename Commands::Symbols → Commands::Bloat

• crates/fbuild-cli/src/cli/args.rs: rename the variant. Add clap
  alias #[command(alias = \"symbols\")] so fbuild symbols ... keeps
  working through 2.3.x (hidden from --help).
• crates/fbuild-cli/src/cli/dispatch.rs: update the match arm.
• crates/fbuild-cli/src/cli/symbols_cmd.rs: rename the module to
  bloat_cmd.rs; rename run_symbols → run_bloat.
• Update crates/fbuild-cli/src/cli/tests.rs and any other call sites.

2. Hide flag dependencies from --help

Today --nm / --cppfilt / --map are advertised. Post-#428 they're
overrides — present but #[arg(hide = true)] so the public surface
becomes just:

fbuild bloat <input> [--build-info <path>] [--json <path>]
            [--output-dir <dir>] [--top N]

3. Make input arg purely a project path

Today the input arg is \"ELF or project directory\". Phase 2 keeps
both forms working, but the doc string + --help text leads with the
project-dir form (which auto-resolves the ELF via
discover_elf_in_project).

4. Error message wording

When build_info.json is missing the new toolchain fields (older
fbuild emitter), error out with:

run fbuild build first so build_info.json carries nm_path

Same shape as the current resolver's error in
crates/fbuild-cli/src/cli/symbols_cmd.rs::run_symbols.

Acceptance

• fbuild bloat . works zero-flag against a fbuild-built project on
  every supported platform.
• fbuild symbols . (legacy alias) still works, prints no
  deprecation warning yet (deprecation lands in 2.4.0).
• fbuild bloat --help does not mention --nm, --cppfilt, or
  --map.

Related

• Meta: fbuild bloat — first-class, end-to-end bloat analysis with build_info-driven toolchain resolution (no manual nm/c++filt, no PlatformIO, no side scripts) #434 (meta).
• fbuild symbols: ingest toolchain paths from build_info.json + extend schema with nm/c++filt/readelf/objdump #428 (Phase 1 blocker, PR feat(symbols): ingest toolchain paths from build_info.json #436).
• feat(symbols): ingest toolchain paths from build_info.json #436 — the schema change this builds on.