0.1.9: merge the impact / path / test-selection batch (#1891, #1894, #1895, #1888, #1893) #1897

Tracks the last batch of impact, path and test-selection work into apps/integration-0.1.9 before the 0.1.9 → dev promotion. Close when all five are merged into the integration branch.

Merge in this order. Each branch already carries the one before it, so later diffs shrink as earlier ones land.

• fix(impact): Python transitive reach — two over-reach sources, Optional class-object params #1891 Python transitive reach: two over-reach sources removed, Optional class-object parameters typed (reach precision 0.675 → 0.724)
• javascript: a test that never loads the change is not selected; a mocha load-time throw selects the whole run #1894 JavaScript test selection: a test that never loads the change is not selected; a mocha load-time throw selects the whole run
• Agent-asserted call edges: list the sites an answer stops at, link them, walk them as [asserted] (#1892) #1895 Agent-asserted call edges (axiomcode link): answers list the sites they stop at; linked sites are walked as [asserted]; links survive rebuilds unless their line is edited (Agent-asserted call edges: list the unresolved sites an answer stops at, and let an agent link them #1892)
• TypeScript: multi-line test registrars, runner set-up files, Object.assign function objects, overload implementations (stacked on #1882) #1888 TypeScript recall: multi-line test registrars, runner set-up files, Object.assign function objects, overload implementations
• TypeScript test selection: leave out tests that never load the change, type tests, package-imported names (stacked on #1888) #1893 TypeScript test selection: tests that never load the change, type tests, package-imported names

Cache versions after the chain: IMPACT_VERSION 74, path EXPORT_VERSION 7 (each PR keeps its own number in the history comment).

Case suites on the final branch (#1893): TypeScript 352/352, JavaScript 354/354, Python 436/436, Java 360/360, C# 265/265.

Not in this batch (next release): the Java and C# direct-call mapping and transitive reach loops, scored against compiler-resolved call graphs.