release: prepare winTerm 1.4.0-alpha prerelease

[HelloThisWorld]

release: prepare winTerm 1.4.0-alpha prerelease

Summary

Switches the source tree from the 1.3.3 engineering checkpoint to the
1.4.0-alpha integration candidate of the Pane Search release, following
the exact preparation pattern of the v1.3.0-beta3 prerelease (PR #42).
After this merges, pushing the v1.4.0-alpha tag on the merge commit runs
the full guarded release pipeline — x64 Release build with compiled tests,
artifact generation, draft creation, asset re-download and
installer/portable round-trip testing, attestation — and only then
publishes the GitHub prerelease (--prerelease --latest=false).
Latest stays v1.2.0; WinGet is not touched.

Version switch (channel stable → alpha)

• applicationVersion 1.3.3 → 1.4.0-alpha; packageVersion
1.3.3.0 → 1.4.0.0; moduleVersion 1.3.3 → 1.4.0 with
  modulePrerelease/PSData.Prerelease alpha; tag → v1.4.0-alpha;
  channel → alpha (both version.json files, appx manifest, all three
  .rc resources in dotted and comma forms, ReleaseMetadata.h with
  ReleaseChannel L"Alpha", psd1/psm1, Workspace descriptor/serializer
  fallbacks).
• custom.props executable metadata advances to major.minor 1.4.
• Pinned literals updated in verify-version.ps1,
  test-visual-progress.ps1, package-shell-assets.ps1, test.ps1
  (including the psd1 prerelease boundary check), verify-branding.ps1.
• The engineering checkpoint tag allowlist is deliberately unchanged:
  v1.4.0-alpha must run the full release pipeline, not checkpoint quick
  validation.

Docs

• docs/releases/1.4.0-alpha.md added (force-added; docs/releases/* is
  gitignored) — it becomes the GitHub release body and the
  winTerm-1.4.0-alpha-release-notes.md asset.
• CHANGELOG gains the 1.4.0-alpha entry; READMEs (EN/JA) now point their
  prerelease link at v1.4.0-alpha (wording updated from the Command
  Timeline beta to the Pane Search alpha) and show the new source version;
  docs/current-progress.md reflects the published-alpha state and the
  PASS→beta / FAIL→1.4.1-alpha flow.

Validation

• verify-version.ps1 (channel-aware: alpha suffix consistency), full
  Smoke suite, test-release-workflow.ps1, verify-branding.ps1 — all
  green locally on this branch.
• The compiled suites were green at the underlying source state
  (Control 89/89, TerminalApp 51/51, SettingsModel 244/244 on v1.3.3,
  which this PR does not modify functionally — version metadata only).

After merge

1. Tag the merge commit v1.4.0-alpha and push the tag.
2. The release workflow builds, tests, drafts, round-trip-tests every
   asset, then publishes the prerelease automatically.
3. Manual validation per the checklist in PR feat: add Pane Search Phase 3 checkpoint (v1.3.3) #46; PASS → 1.4.0-beta,
   FAIL → 1.4.1-alpha.