engine: surface math the go-tex/math layer drops (Diagnostics.MathDropped)

[tannevaled]

Instrumentation #3 — surface MATH content the math layer drops

Feasibility finding

The go-tex/math parser does not silently swallow unknown control words: parse.go returns a hard error texmath: unknown command \X (and other structural errors like \left without \right). The engine already catches these in renderMathResolvingMacros/makeMath, and in lenient mode drops the whole equation and tallies the trigger via recordMathSkip. So go-tex/math already exposes unhandled tokens through its error return — bullet 1 of the task applies, and no change to the go-tex/math module is needed.

The one gap: that tally landed in the same skippedCS map as text-mode undefined commands, so a formula's silently-lost content was conflated with a missing text macro. In -report-skipped a math key (which carries its own \) was even printed with a doubled backslash.

(The only genuinely-silent degradations in go-tex/math are within array/column-spec parsing — an unknown column letter falls back to centred, stray punctuation is ignored. These are rendering approximations, not content drops, and are not worth a math-module change.)

Approach chosen

Separate math drops from text skips, without touching rendering or the existing skippedCS surface:

• recordMathSkip now also tallies into a dedicated mathDropped map[string]int (same key).
• Diagnostics() lifts those keys out of Skipped into a new MathDropped field (added at the end of the struct — trivial merge). Text-mode gaps stay in Skipped; math-layer drops go in MathDropped.
• Raw skippedCS is unchanged, so SkippedCommands() and every existing caller/test keep identical behaviour.
• Surfaced in the -report-skipped CLI report (its own "math equation group(s) dropped" section) and the wasm diagnostics object (mathDropped).

Tests (RED without / GREEN with)

mathdropped_test.go + a CLI TestRunReportMathDropped:

• an unknown math command surfaces under MathDropped and not under Skipped (proving the separation), while SkippedCommands() still carries it;
• a malformed equation (\left( unclosed) is counted under the $math$ sentinel;
• valid math (\frac, \sum, \cdot, \alpha) drops nothing;
• the snapshot is a copy.

Verified by temporarily disabling the tally: both new tests go RED (the report even reproduced the old doubled-backslash conflation), GREEN once restored.

Gates

• GOWORK=off go test -race ./... green
• gofmt -l clean, go vet ./... clean
• GOWORK=off GOOS=js GOARCH=wasm go build ./... green
• engine package coverage 92.5% (cmd/gotex 73.0%→73.6%, a pre-existing infra gap; new code paths covered)

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