Broadcast parent-side line-text edits to active tool iframes

[thehabes]

Surfaced during testing of the 5-2-tools-align cut (PR #564). The parent transcription-block does not broadcast text-input changes to the active tool iframe, so iframe tools that display line text (notably Preview-Transcription) show only the values fetched at boot, plus values they themselves sent via UPDATE_LINE_TEXT. Edits typed directly into the parent stay invisible to the tool until reload.

This is a pre-existing gap, not introduced by #564. Deferred so the contract-standardization PR stays focused on standardization.

Proposed shape

Extend the parent → tool contract with one new message:

{ type: 'LINE_TEXT_UPDATED', lineId: '<URI>', text: '<string>' }

Wire transcription-block's input-change handler (around the existing markLineDirty / scheduleLineSave sites in components/transcription-block/index.js) to call simple-transcription's #postToTool with the new message. The active iframe's #activeToolIframe is already tracked in simple-transcription.

Companion work

Preview-Transcription needs a corresponding inbound handler. Tracked separately so each repo carries its own commit.

Companion issue

CenterForDigitalHumanities/Preview-Transcription#7