F-101: close target() deps API audit + docs

[stepango]

Summary

Closes F-101 / GH #56: audit and lock target(...) as the single happy path for internal module deps.

Code

• Pure ProjectPathForms.gradleProjectPathFromFormaTarget — Forma colon path → Gradle/includer dashed path (:feature:home:impl → :feature-home-impl)
• Project.target(String) wired through the helper (parity + clearer rejects for empty / missing :)
• Unit tests on :core (ProjectPathFormsTest)

Docs

• DEPS-CATALOG.md §3 Project / target deps (API table, path forms, rejects)
• CALL-SITE-SURFACE.md short F-101 section
• GETTING-STARTED cross-link; README example uses target(...) not raw project(...)

Locked decisions

Concern	Choice
Happy path	target(":a:b:c") colon Forma paths + typesafe accessors
Reject	raw project() in module deps as happy path
Out of scope	slash/Bazel paths (discussion #57)

Verify (host)

source scripts/env-mac.sh
cd plugins && ./gradlew :core:test --tests '…ProjectPathFormsTest' :deps:test jacocoHappyPathCoverageVerification
→ BUILD SUCCESSFUL

Closes #56