Closes#5992
## Summary
Expose the existing **Text content** binding control when a directly
text-capable selected instance contains expressions alongside other
content.
The supplied reading-time span keeps its existing child sequence:
- static prefix text
- bound expression
No data migration or new Settings editor is introduced. Normalization
happens only when the user explicitly resets a binding.
Expression-bearing nodes are excluded from canvas text editing because
the canvas editor cannot represent bindings safely. Literal descendants
in mixed trees remain canvas-editable, while bindings remain editable
through Settings.
## Implementation
- Reuse the existing scalar Text content and binding UI.
- Target expression children with indexed `instances.updateText`
mutations, preserving instance boundaries and unrelated siblings.
- Fail closed when the selected instance is missing or no longer has a
safe text target.
- Evaluate expressions through the binding control's existing live
scope.
- Centralize text-content target classification in project-build for
reuse by the engine and Builder.
- Reset every expression atomically through the engine's
`instances.setTextContent` operation, validate the complete replacement
set, and coalesce adjacent text nodes without crossing nested-instance
boundaries.
- Centralize expression-free canvas text-editor targeting in the
project-build lookup module.
- Route double-click, Enter, post-insert editing, cursor generation, and
keyboard navigation through the shared lookup rules.
- Exclude single-expression and mixed/nested expression targets from the
canvas editor in every mode.
- Preserve canvas editing for literal descendants inside otherwise mixed
content.
- Share canvas fixtures and simplify binding-removal capability to an
optional handler.
## Verification
- [x] Settings exposes and edits mixed text/expression targets without
replacing siblings.
- [x] Reset binding converts every expression on the instance and
normalizes adjacent text.
- [x] Missing, unsupported, stale, and incomplete replacement sets fail
closed.
- [x] Double-click and Enter do not open bound nodes in the canvas
editor.
- [x] Literal descendants within mixed trees remain canvas-editable.
- [x] Complete Builder suite passes (2,497 tests).
- [x] Complete project-build suite passes (2,070 tests).
- [x] New targeted Builder and project-build regression tests pass (175
tests).
- [x] Builder and project-build typechecks pass.
- [x] Repository lint passes.
- [x] Package-boundary validation passes.
- [x] Generated runtime operation contracts are updated from source.
- [x] Fixture inputs and outputs are unaffected.
- [x] Documentation impact reviewed: no documentation update is needed.
- [ ] Interactive browser smoke test was unavailable because the in-app
browser failed to initialize.
- [ ] Agent evaluations were not run because they require separate
approval.
## CI note
The previous head's Main workflow failed while pulling the E2E database
image with Docker Hub `toomanyrequests: Rate exceeded`. Lost Pixel
failed separately with a certificate error. Both are external
infrastructure failures; branch formatting, lint, package boundaries,
generated contracts/docs, tests, and typechecks were successful in that
run.
Supersedes #5993, which GitHub closed automatically when its head branch
was renamed.
Oleg Isonen