fix(flow): refuse invisible-only text selectors — record coordinates instead

[filip131311]

The bug (found in the 0.15.0 full-surface QA, on both the Android emulator and a physical S24)

Recording a gesture-tap on an icon-font tab-bar item wrote a flow step that displays as an empty selector, with the coordinates gone:

steps:
  - tap:
      text:      # ← actually U+E163, an icon-font Private Use Area glyph

and flow-execute then silently passed against target "", tapping an element no reader of the flow can predict. Root cause (hexdump of the QA-recorded file: ee 85 a3 = U+E163): icon fonts expose their glyph as the element's accessibility label; deriveSelector accepted any label that survived trim(), so a PUA code point counted as "stable text". This violated the rewrite's own documented contract — "otherwise coordinates are kept with a warning" — whose fallback path existed and worked, but never ran for this case.

The fix — one predicate, three layers

hasVisibleText() (strip Unicode Cf format / Co private-use / Cc control, then trim; require something left):

1. deriveSelector — invisible-only label/value is not stable text; fall through to role, then to the recorder's existing kept-coordinates path.
2. selectorToYaml — the empty-text guard previously covered only the bare-string spelling; it now also rejects empty/invisible text on the strict {text: …} map form, so a hand-built selector fails loudly instead of writing an unreadable flow file.
3. selectorSchema — parse-side refinement, so flow files already recorded broken on 0.15.0 fail with an actionable message ("text must contain at least one visible character … select by identifier or role instead") instead of false-passing at replay. This also hardens the await-ui-element tool surface, which shares the schema.

Text that merely contains an icon glyph alongside real text ("⟨U+E163⟩ Explore") stays valid throughout.

Verification

• Unit: deriveSelector PUA/ZWSP/mixed cases, serialize + parse rejection round-trips (explicit \uXXXX escapes, no invisible bytes in the test source).
• Full tool-server suite: 2667 passed.
• Live on the Pixel_9 emulator against the original QA repro (my-app tab bar, same coordinates): recording now produces tap: {x: 0.75, y: 0.947} with the documented "kept coordinates (brittle)" warning — the role fallback was correctly caught by the existing resolves-to-a-different-element guard — replay passes 2/2 on the coordinate targets, and executing the old broken YAML now fails parse with the visible-character error.

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