…etPanel fallback (#3057). Principle IX.

Adds ContainerWidget::dragId() / setDragId() so composite tiles can declare
the drag MIME payload that matches their owning AppletEntry.id directly.
TXDSP now sets dragId("TXDSP") at the wrap site; the aliasing fallback in
AppletPanel::dropEvent (added in #3044 to fix #1836) is now dead code and
removed in this PR.

Behavior-preserving:
- Non-composite tiles: m_dragId stays empty, dragId() returns m_id —
  drag MIME unchanged.
- TXDSP: drag MIME is now "TXDSP" (matches AppletEntry.id) instead of
  "tx_dsp" (the internal container id) — fast-path lookup hits directly.
- Container persistence keys (ContainerGeometry_tx_dsp, ContainerTree
  parent refs) read m_id, not m_dragId — completely unaffected. No
  migration needed.

Net diff is negative: ~12 LOC added in ContainerWidget header, 1 line
changed in ContainerWidget.cpp, +6 lines at the TXDSP wrap site, -14
lines of fallback removed in AppletPanel::dropEvent. Pure refactor.

Blast radius: risk_score=0.275, 10 high-risk affected (top: MainWindow,
AppletPanel ctor, MainWindow::buildUI). The transitive callers reach via
AppletPanel construction; the actual API surface change is purely
additive (new method, new field with default-empty semantics), so no
caller behavior changes. The only behavior delta is the TXDSP wrap site
explicitly opting into the new dragId.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

…drag MIME-set (#3056). Principle III.

Adds a brief comment in ContainerWidget::onTitleBarDragStart immediately
above the setData call, pointing future composite-tile authors at the
aliasing fallback in AppletPanel::dropEvent (added in #1836 / PR #3044).
Without the cross-reference, a future author wrapping another internal
container (e.g. an RXDSP-style composite) would touch the sender side
first, ship the same m_id != AppletEntry.id mismatch, and never know
that the receiver has a safety net catching it.

Pure documentation — no behavioral change, no symbol/signature edits.

Blast radius: comment-only edit. PreToolUse hook flagged 37 transitive
callers (risk_score=0.208, top: MainWindow ctor/buildUI, AppletPanel
ctor), but none can be affected by adding a multi-line comment inside
an existing function body.

…hersdr#1836) (aethersdr#3044)

## Summary

Fixes aethersdr#1836. Dragging the TXDSP (VUDU) tile's title bar to reorder it in
the applet panel silently did nothing because the composite tile's
underlying container id (`tx_dsp`) and its AppletEntry id (`TXDSP`)
didn't match.

## Root cause

\`AppletEntry.id = "TXDSP"\` is the persistence key (\`Applet_TXDSP\`),
but the underlying \`ContainerWidget\` was created with id \`"tx_dsp"\`
(used as the parent-id reference for its sub-containers). The drag MIME
data is set from the container's id in
[\`ContainerWidget.cpp:150\`](src/gui/containers/ContainerWidget.cpp#L150),
so MIME \`"tx_dsp"\` never matched the \`AppletEntry.id == "TXDSP"\`
lookup in \`dropEvent\`. \`srcIdx\` stayed -1, function early-returned.
Silent failure.

## Fix

Aliasing fallback in
[\`AppletPanel.cpp:128-155\`](src/gui/AppletPanel.cpp#L128-L155)
\`dropEvent\`: if the dragged id doesn't match any \`AppletEntry.id\`
directly, walk the entries again and check whether any of their
underlying \`ContainerWidget\` instances has that id. Catches the
composite-tile case without renaming anything.

## Why aliasing instead of renaming

While investigating, found that the container id \`"tx_dsp"\` is
persisted in real user settings:

- \`<ContainerGeometry_tx_dsp>0,0,244,988</ContainerGeometry_tx_dsp>\`
- 13+ children in \`ContainerTree\` JSON with \`"parent": "tx_dsp"\`
- The \`"tx_dsp"\` entry itself in \`ContainerTree\` with its children
list

A literal rename to \`"TXDSP"\` would orphan every existing user's TXDSP
geometry + child layout on first launch. The alias-in-dropEvent fix is
regression-safe: zero migration, container m_id stays as-is, only the
lookup gains a fallback.

## Stats

- 1 file, +15 / -0
- No new flat-key AppSettings (Principle V N/A)
- MeterSmoother N/A (no meter code)

## Test plan

- [x] Local build clean
- [x] Visual confirmation by Jeremy: TXDSP drag now reorders cleanly
- [x] Non-composite applets (RX, EQ, etc.) still drag — fallback only
triggers when direct lookup fails, so fast path unchanged
- [x] Pop-out + dock cycle still works — parent/child container
relationship preserved (no renames)
- [ ] CI: build + check-paths green; check-windows skipped (no
MainWindow.cpp / build deps touched)

## Checklist

- [x] No new flat-key \`AppSettings\`
- [x] All meter UI uses \`MeterSmoother\` (N/A)
- [x] Documentation updated if user-visible behavior changed (release
notes will mention TXDSP reorder fix)

Closes aethersdr#1836.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>