fix(dylint): give each boundary-lint process a unique observation identity

[zackees]

Symptom

Dylint (windows) failed on #1379 — a change confined to fbuild-cli:

platform-boundary: actual Dylint observations disagree for pid=2636
  source=crates/fbuild-toolchain/src/toolchain/esp_qemu.rs:
  expected={('attr_cfg', 'windows'): 6} actual={('attr_cfg', 'windows'): 12}
platform-boundary: actual Dylint observations disagree for pid=2636
  source=crates/fbuild-toolchain/src/toolchain/esp_qemu_runtime.rs:
  expected={('attr_cfg', 'target_os'): 3} actual={('attr_cfg', 'target_os'): 6}

Three things pointed away from the PR: nothing in it touched
fbuild-toolchain, both counts were exactly double, and the Linux leg of the
same commit was green.

Cause

dylint --all-targets runs one driver process per crate-target, so a crate's
lib and lib-test targets compile the same sources twice. Each process stamped
its observation lines with std::process::id() and nothing else. Windows
recycles PIDs readily enough that the second process can be handed the first
one's number once it has exited — and compare_dylint_observations keys
findings by (process, source, kind, normalized), so the two runs merged into
one bucket and every count for the sources they share read double.

The same collision is possible on Linux; the PID space is large enough that it
effectively never happens, which is why the gate has looked stable.

Verification

Replaying a real observation file with every identity collapsed to one PID
reproduces the CI failure exactly — the same two files, the same 6 → 12 and
3 → 6:

$ awk -F'\t' 'BEGIN{OFS="\t"} {$1="2636"; print}' observed.tsv > collide.tsv
$ uv run --no-project python ci/enforce_platform_boundary.py --dylint-observed collide.tsv
platform-boundary: actual Dylint observations disagree for pid=2636 source=…/esp_qemu.rs: expected={('attr_cfg', 'windows'): 6} actual={('attr_cfg', 'windows'): 12}
platform-boundary: actual Dylint observations disagree for pid=2636 source=…/esp_qemu_runtime.rs: expected={('attr_cfg', 'target_os'): 3} actual={('attr_cfg', 'target_os'): 6}

With the fix in place the same sweep emits distinct identities and the checker
passes:

$ cut -f1 target/platform-boundary-dylint-observed.tsv | sort -u
13176-18ce7fb775e64928
17564-18ce7fb7cba933c0
25424-18ce7fb88715b32c
…
$ uv run --no-project python ci/enforce_platform_boundary.py --dylint-observed … --print-totals
rows=14; dylint_rows=14

Fix

The identity is <pid>-<nanos>, taken once per process via a LazyLock.
Reusing a PID requires the original holder to have exited first, so two
processes cannot share a start nanosecond.

The Python checker needs no change — it already treats the first field as an
opaque key. Deduplicating findings would have been the wrong fix: the counter
legitimately counts repeated (kind, normalized) pairs within one file.

Crate version bumped so the cached .so is rebuilt with the new emitter.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between b6f7eb9 and 8476651.

⛔ Files ignored due to path filters (1)

• dylints/enforce_platform_boundary/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (2)

• dylints/enforce_platform_boundary/Cargo.toml
• dylints/enforce_platform_boundary/src/lib.rs

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