fix(producer,engine): stop mislabelling capture mode, and name the silent drawElement refusals

[vanceingalls]

Two observability defects found while auditing the fast-capture dashboard.
Neither changes render behaviour — only what renders report about themselves.

1. captureMode reported beginframe on hosts that cannot run it

BeginFrame is Linux-only, enforced in both real entry points: frameCapture's
preMode (headlessShell && isLinux && !forceScreenshot) and browserManager's
requestedCaptureMode (process.platform === "linux"). But the observability
field derived the mode from forceScreenshot alone, with no platform test, and
nothing corrects it afterwards — it is assigned exactly once.

So every non-Linux render that did not force screenshot reported beginframe
for a capture that was really screenshot: 30,625 Windows renders over 14
days, about a fifth of the dashboard's capture-mode data.

config.ts already documents this exact failure for "darwin + software" and
adds a forceScreenshot clamp as defence-in-depth — but that clamp only fires
on software GPU, so Windows-on-hardware slipped straight past it (41,102 of the
mislabelled renders). Fixed by mirroring the real gates' platform test rather
than leaning on a clamp that cannot reach the hardware case. Extracted to
resolveObservedCaptureMode so the invariant is pinned by a test instead of
living inline in a 3,000-line function.

distributed/plan.ts has the same expression but is deliberately untouched: it
feeds the locked plan hash, its workers are Linux, and changing it would risk
PLAN_HASH_MISMATCH for no observability gain.

2. Renders that never became drawElement candidates had no reason at all

Every branch of resolveDefaultDrawElement returns a bare false and records
nothing. The orchestrator's clamp only runs if (cfg.useDrawElement && ...),
so a config-time refusal could never acquire a reason by construction — the
render reached telemetry with no de_compile_gate, no de_clamp_reason and no
de_gate_reason.

Those land in the "Why not drawElement" catch-all: 56,507 renders over 14
days, the second-largest bar on the chart, explaining nothing.

Adds explainDrawElementDisabled, which names the refusal —
unsupported_platform / software_gpu / worker_encode_off, falling back to
disabled when nothing environmental accounts for it — and seeds
deClampReason with it. Later clamps still overwrite: a more specific reason
wins.

It takes only the environmental inputs deliberately. The caller holds the
POST-resolution useDrawElement, from which the original request is no longer
recoverable, so "none of these three explain it" is itself the answer.

Tests

Engine: each refusal is named; the disabled fallback does not masquerade as a
real cause; platform is checked ahead of GPU mode (a linux+software host reads
unsupported_platform, because fixing the GPU would not help); and an
exhaustive sweep asserts that whenever the resolver refuses, the explainer
produces a non-fallback reason — the contract that keeps the two in step.

Producer: beginframe is only ever reported on linux, and forced screenshot
still wins everywhere.

engine 1480 passing, producer 579 passing. oxlint and oxfmt clean.