chore: validate JetBrains plugin in a live IDE (runIde + Plugin Verifier) on an unrestricted network

[onokonem]

Follow-up from #29.

The plugin (8cefec6 + 34d6aa5) builds and packages correctly — Gradle build + verifyPluginStructure green, buildPlugin produces an installable zip, 38/38 pure-JVM tests pass — but download.jetbrains.com returns HTTP 451 from the network where it was developed, so two validation steps could not run there:

1. runIde smoke test in a live IDE 2025.3+: open a .c4d fixture from examples/, confirm diagnostics match the CLI, completion/hover/definition fire, formatting works, and the "C4Drill Preview" tool window renders with working drill-through (JCEF click bridge → c4drill/renderDiagram).
2. Cross-version Plugin Verifier (verifyPlugin) against the intended compatibility range (since-build 253 / 2025.3+, all commercial IDEs).

The build supports a local IDE via C4DRILL_IDEA_HOME (env var → local(); unset falls back to canonical intellijIdeaUltimate("2025.3")) — documented in idea-plugin/README.md.

Acceptance

• runIde manual pass: LSP features + preview verified interactively on 2025.3
• verifyPlugin clean across the declared compatibility range
• Any API deprecations/_usages the verifier flags are fixed
• Marketplace listing checklist (plugin icon, description, change notes) if publishing is intended

[onokonem]

Automation scope complete (885d8a3 + 2b96ea4). Evidence:

• Plugin Verifier across the bracketing ends of the compatibility range: IU-253.28294.334 (2025.3) and IU-262.9437.185 (2026.2.1) — both Compatible, zero fatal. Note categories (deprecated/experimental) documented in idea-plugin/README.md as deliberately kept: the Kotlin compiler-generated jvm-default bridges aren't source-level calls, and the 262 com.intellij.platform.lsp.api deprecation's replacement only exists on 262+ (unusable while since-build is 253).
• One real fix the verifier forced: C4drillFormatAction invoked AnAction.actionPerformed directly (@ApiStatus.OverrideOnly — the only fatal category). Now routes through ActionUtil.performActionDumbAwareWithCallbacks + ACTION_EDITOR_REFORMAT; re-verified Compatible on both IDEs.
• 48/48 tests (38 pre-existing + 10 new headless platform fixtures against the real 2025.3 test distribution: file-type registration, TextMate bundle extraction + real tokenization, settings/TOML scoping on VirtualFiles, gateway EP wiring). Stability checked across 4 consecutive runs. Headless ceiling constraints documented, tests shaped to them rather than faked.
• LSP end-to-end probe with the real binary over stdio: initialize → full capabilities; broken model → publishDiagnostics with the CLI-identical message; c4drill/renderDiagram → 18 KB SVG for examples/cloud-system/cloud-system.toml.
• Marketplace checklist: icon/description existed; change notes added.

Remaining (needs a human): the interactive runIde pass — preview tool window, JCEF drill-through, live diagnostics watch. A 20-step manual QA script with exact steps and expected results is appended to idea-plugin/README.md; the issue stays open until that pass is done.