Add cmake_profile backend (configure-step profile, closes BuildDigest)

[onlyxItachi]

Summary

New backend: cmake_profile digests cmake --profiling-format=google-trace
output (cmake >= 3.18) — where the CONFIGURE step spends time (find_package
probes, try_compile, include chains, user macros/functions). Domain reuses
build_phase (a configure frame is a build phase; the real data gave no
reason for a new constant). This closes the BuildDigest family:
ninja_log = build edges, clang_time_trace = compiler phases,
cmake_profile = the configure step itself.

The real-report finding that shaped the design

The task assumed straightforward chrome-trace reuse. Inspecting the real
capture first (installed cmake 4.3.2) showed cmake emits zero ph=='X'
complete events — the trace is a bare JSON array of ph=='B'/'E'
begin/end PAIRS (4523 each on the cold run), and the E side carries only
ph/pid/tid/ts. Naive _grouped reuse would silently digest every cmake
trace to zero units.

Fix: a fold_be_pairs hook in chrome_trace's _complete_events/_grouped —
the same surgical-hook pattern tag_override established for
clang_time_trace. B/E pairs are stack-folded per (pid, tid) (Chrome-trace
semantics: E closes the most recent open B) into synthetic complete events
taking name/cat/args/ts from the B side, dur = E.ts - B.ts. Default False
leaves chrome_trace/clang_time_trace behavior byte-for-byte unchanged (full
suite proves it). Honesty: a leftover B from a truncated trace is dropped —
never given a fabricated dur (tested); a stray E likewise.

Grouping — decided from the real data

Every B event is cat: "script" with name = the cmake command/macro/
function that ran; the per-call detail lives in args
(functionArgs, location). Chosen: (cat, name) aggregation — the
sibling backends' rule — which turned 4523 spans into 83 units on the cold
run, with hot units directly actionable (try_compile 99ms and
find_package 55ms of a 223ms cold configure). Rejected: per-call-site
grouping by args.location (unit-count explosion, host paths in unit
names). The call-site detail stays reachable per-unit via expand
(arg:location, arg:functionArgs) — tested.

Usage notes carry the family overlap caveat: frames NEST (configure and
generate are cmake's whole-step frames containing everything else), so
total_time_us overlaps across units and configure ranking first is
expected, not a hotspot.

Fixture provenance (real, not hand-fabricated)

tests/fixtures/cmake_profile_sample/ is the committed project
(CMakeLists.txt with find_package(Threads), an include()d helper defining
a user function(), a static lib + executable, a foreach of messages),
profiled with the installed cmake 4.3.2 in a neutral temp dir. Two runs:

• run 1, cold configure: 1.5MB / 9046 events, dominated by compiler-detection
  try_compile — used for design verification, not committed.
• run 2, warm RE-configure (the everyday "why is my configure slow"
  artifact): 1598 events — committed as
  cmake_profile_sample.cmake-profile.json.

Host-path hygiene, exactly as the task anticipated: 86 args.location values
pointing into the temp source/build dirs were relativized to
proj//build/. Verified programmatically: every field OUTSIDE args
(names, ph, ts, pid, tid) is byte-identical to the raw capture; /usr/share/ cmake-4.3/Modules/... locations are verbatim; timings untouched; zero
leaks (grep for user/home/tmp paths clean).

Test evidence

New file tests/test_cmake_profile.py, 10 tests, all passing:

• registry dispatch on cmake-profile/cmake-trace (+ case-insensitive)
• real-trace parse from B/E pairs (50 units incl. configure, generate,
  find_package, the user function add_tagged_executable)
• name aggregation across call sites (include calls=28, total 6132us,
  max 1091us — exact committed values)
• ranking (configure 10166us > include > project)
• nested-overlap proof on real data (sum of units >> configure + generate)
• expand reaches arg:location/arg:functionArgs (relativized, no host
  paths) and per-call durs_us
• truncated-trace honesty: unpaired B vanishes, never a fabricated duration
• inherited loud errors on non-JSON and JSON-without-traceEvents
• absence honesty + vocabulary hint for an out-of-domain metric
• cross-backend smoke check that the fold hook (default off) left the torch
  fixture's digestion intact

Full suite: uv sync --extra dev && uv run --extra dev pytest -q ->
178 passed, 16 skipped (baseline on main was 168 + 16; +10 new tests,
zero regressions — chrome_trace/clang_time_trace suites all green after the
shared-reader hook).

Scope discipline

ONE import line in server/app.py, ONE PROFILER_TOOLS entry
("cmake_profile": "cmake"), no new domain constant (reused
DOMAIN_BUILD_PHASE per the task default), plus the fold_be_pairs hook in
adapters/chrome_trace/trace_reader.py following the established
tag_override precedent of extending the shared trace machinery for a sibling.
No version bump, no README/CLAUDE.md edits. Probe stays exec-free like every
sibling (the >= 3.18 requirement is in the probe notes; an older cmake
rejects the flag loudly, so nothing can go silently missing).

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