fix(player): reject non-finite composition dimensions from attributes and stage-size

[calcarazgre646]

Problem

Two of the three paths that set the player's composition dimensions skip validation, and both feed scaleIframeToFit()'s w / compositionWidth division (iframe-dom.ts:70):

1. width/height attributes (hyperframes-player.ts:174-179): raw parseInt(val || "1920", 10). A typo in the embed like width="abc" produces NaN, which lands in the iframe as an invalid scale(NaN) transform plus width: NaNpx; width="0" produces a division by zero (scale(Infinity)); negative values flip the iframe. In every case the player renders blank with no signal about why.
2. stage-size runtime message (runtime-message-handler.ts:88-93): the > 0 check alone lets Infinity through (Infinity > 0 is true), which scales the iframe to 0 and blanks it.

The third path already validates: the composition probe rejects non-finite and non-positive dimensions via readPositiveDimension() before they reach the same sink. And the inconsistency is local in both cases — the timeline branch directly above the stage-size check does finite-check its input, and playback-rate in the same attribute switch goes through clampPlaybackRate (#1120).

Change

• Export readPositiveDimension from composition-probe.ts and use it for the width/height attributes, falling back to the 1920/1080 defaults the val || ... pattern already implied.
• Add the same Number.isFinite guard the adjacent timeline branch uses to the stage-size check.

No behavior change for valid inputs: finite positive dimensions parse exactly as before.

Tests

• Attribute path: valid values apply; non-numeric, zero, and negative fall back to defaults; removing the attribute restores defaults. The non-numeric and zero cases fail against the previous code.
• stage-size path (new runtime-message-handler.test.ts): finite positive sizes apply; Infinity/NaN/zero/negative/string sizes are ignored (the Infinity cases fail against the previous code); messages from a different source window are ignored.
• Full player suite: 132 passed. bun run build green.