feat: add StartProcessByMessageAtElementCmd to process API

[emaarco]

What

Adds StartProcessByMessageAtElementCmd to the Process API — analogous to the existing StartProcessByDefinitionAtElementCmd, but it starts a process at a specific element via a message name instead of a definition key.

This enables starting at a given element for processes that have no plain (none) start event and are entered only through one or multiple message start events — the use case described in #292.

Changes

• New command api/.../process/StartProcessByMessageAtElementCmd.kt (messageName, elementId, payload, restrictions), with the same Java-friendly constructor overloads as its definition-based sibling. Tagged @since 1.7. No dispatch changes needed — StartProcessApi.startProcess(...) accepts it polymorphically.
• Docs docs/process-api.md: intro now lists four start variants and includes a startByMessageAtActivity(...) example.
• README feature matrix: new Start Process by message at row + an icon legend.

Adapter support

The matrix marks the new feature 🚧 (in development) for C7 embedded, C7 remote and CIB7, and ❌ for C8.

C8/Zeebe is not technically supportable: CreateProcessInstance with startInstructions starts by process definition id (not message), and PublishMessage has no startInstructions — the two cannot be combined, and there is no API to resolve a message name to its owning definition.

Adapter implementations are tracked in separate tickets and are out of scope for this PR:

• CIB Seven — Implement StartProcessByMessageAtElementCmd (CIB Seven embedded) process-engine-adapters-cib-seven#73
• Camunda 7 — Implement StartProcessByMessageAtElementCmd (Camunda 7 embedded + remote) process-engine-adapters-camunda-7#199
• Camunda 8 — Investigate & implement StartProcessByMessageAtElementCmd (Camunda 8) process-engine-adapters-camunda-8#233

Docs delivery

The published docs site (process-engine-api-docs) imports docs/**/*.md from this repo's develop branch at build time via the mkdocs-multirepo plugin, so no separate docs-repo PR is needed. The site rebuilds on its own push/tag (left to the release).

Refs #292

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 49.67%. Comparing base (f791d56) to head (4740491).

Files with missing lines	Patch %	Lines
...neapi/process/StartProcessByMessageAtElementCmd.kt	95.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #293      +/-   ##
=============================================
+ Coverage      47.65%   49.67%   +2.02%     
- Complexity        72       80       +8     
=============================================
  Files             47       48       +1     
  Lines            447      467      +20     
  Branches           9        9              
=============================================
+ Hits             213      232      +19     
- Misses           229      230       +1     
  Partials           5        5              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.