feat(features): Stage 2a — feature-activated optional dependencies (v0.0.71)

[Sunrisepeak]

What

A dependency declared under a feature is pulled into resolution only when that feature is active. Optionality is expressed by where you declare it — no optional = true flag.

[features]
use_blas         = { defines = ["EIGEN_USE_BLAS"], requires = ["blas"] }
backend-openblas = { implies = ["use_blas"] }

[feature-deps.backend-openblas]   # pulled ONLY when backend-openblas is active
compat.openblas = "0.3.x"

This composes with Stage 3 capabilities: features = ["backend-openblas"] pulls the provider (compat.openblas, which provides = ["blas"]) and turns on the consumer switch (implies use_blas, which requires blas) → the capability binds automatically. One-line backend selection.

How

• Manifest.featureDeps : map<feature, map<depKey, DependencySpec>>. Parsed from TOML [feature-deps.<name>] (reuses the dependency loader) and from a Lua descriptor feature's nested deps = { ["name"] = "ver" }. Lua feature implies is now parsed too (was TOML-only).
• prepare_build: two local lambdas activateFeatures / mergeActiveFeatureDeps merge a manifest's active feature-deps into its dependencies map — root before the worklist seed, each dep right after its manifest loads. The existing worklist BFS then resolves them; no new resolution phase, no re-entrancy. Optional-by-default falls out for free.

The two helpers MUST be local lambdas, not file-scope functions: as exported (inline) functions in this module-interface unit their std::map instantiations leak into the BMI and trip a GCC-16 modules bug — another TU importing std then fails with failed to load pendings for __normal_iterator. (Diagnosed by bisecting: build is clean without these helpers; fails when they're file-scope.)

Deferred

S2b — feature unification (union feature requests per package identity across the graph; today first-requester wins) is the next stage — the one genuine resolver-semantics change.

Tests

• e2e: 82_feature_optional_deps.sh (off → dep not pulled; on → pulled + compiled).
• unit: Manifest.FeatureDepsTomlSection, SynthesizeFromXpkgLua.FeatureDepsAndImplies.
• Full feature e2e (67/71/80/81/82) + unit suite green locally.

Design: .agents/docs/2026-06-29-feature-optional-dependencies-s2-design.md.