Document configurable module roots as next phase

docs/BUILD_PLAN.md

@@ -169,7 +169,8 @@ Each phase has a machine-checkable gate. Do not advance without a green gate.
 | **2** | `SyntaxHighlighter`, color settings page, commenter, brace matcher | Tier 1 golden dumps; highlighter maps every token type |
 | **3** | `.bnf` grammar, generated parser + PSI, `ParserDefinition` | Tier 2 golden trees; Tier 3 corpus parse ≥ target — **done, 100.0% (714/714)** |
 | **4** | Structure view, folding, `#import`/`#load` reference resolution + go-to-definition | Tier 4 fixture tests — **done** |
-| **5** | Completion (keywords, directives, module names), rename, find-usages | Tier 4 fixture tests — **done** (70-test suite) |
+| **5** | Completion (keywords, directives, module names), rename, find-usages | Tier 4 fixture tests — **done** (71-test suite) |
+| **5a** | Configurable Jai module/import roots and indexed external search scope | Tier 4 settings, resolution, completion, navigation, and find-usages tests |
 | **6** | Formatter, code style settings | Formatter round-trip: formatting the corpus is idempotent |
 | **7** | Inspections (e.g. `#must` misuse), quick fixes, live templates | Tier 4 + `verifyPlugin` |
 | **8** | Optional: run-configuration to invoke the `jai` compiler, parse its error output | Integration test against `~/.local/jai/bin` |
@@ -207,6 +208,7 @@ corpus-scale invariant that needs no human judgment.
    setup, works today), or install a system JDK via Homebrew? I'd default to the
    wrapper.
 
-Phases 0–5 are now implemented and verified headlessly. The remaining decision
-is whether to continue with the optional formatter, inspections, and compiler
-integration work in Phases 6–8.
+Phases 0–5 are implemented and verified headlessly. The next planned increment
+is Phase 5a: configurable module/import roots and indexed external search scope.
+After that, decide whether to continue with the optional formatter, inspections,
+and compiler integration work in Phases 6–8.