refactor: den-fx

[sini]

Summary

Replace den's legacy recursive tree-walking resolution with an effects-based pipeline. Aspects compile into effectful computations via aspectToEffect — the tree structure emerges from effect composition, not explicit recursion. All resolution strategy (constraint checking, dedup, tracing, context dispatch) lives in handlers.

• aspectToEffect compiler — single function compiles any aspect into a computation that emits emit-class, register-constraint, emit-include, and resolve-complete effects
• Handler-owned recursion — emit-include handler checks constraints and recurses via effectful resume; into-transition handler processes context transitions with scope.stateful
• Constraint system — meta.handleWith / meta.excludes replace fx usage of meta.adapter; scoped constraints via includes-chain ancestry; exclude, substitute, filterBy constructors with subtree/global variants
• Includes chain provenance — chain-push/chain-pop effects replace __parent string tracking; observable by any handler for diagrams, scope visualization, composable analysis
• Module wiring — { lib, den } only, no init function, barrel default.nix; nix-effects accessed as den.lib.fx
• den.fxPipeline option — defaults to true; legacy tests using resolve.withAdapter run with fxPipeline = false

Pipeline architecture

nix/lib/aspects/fx/
  aspect.nix        — aspectToEffect compiler
  pipeline.nix      — mkPipeline, defaultHandlers, fxResolve
  identity.nix      — path/identity utilities, collectPathsHandler, pathSetHandler
  constraints.nix   — exclude, substitute, filterBy constructors
  includes.nix      — includeIf conditional inclusion
  trace.nix         — structuredTraceHandler, tracingHandler
  handlers/
    include.nix     — emit-include handler (owns recursion)
    transition.nix  — into-transition handler (scope.stateful)
    ctx.nix         — constantHandler, ctxSeenHandler
    tree.nix        — constraintRegistryHandler, chainHandler, classCollectorHandler

Effect protocol

Effect	Handler responsibility
emit-class	Accumulate modules by class
emit-include	Check constraints, recurse via aspectToEffect
register-constraint	Store in registry with scope/ownerChain
check-constraint	Query registry, first-registered-wins
chain-push / chain-pop	Track includes-path stack
into-transition	Walk context transitions with scoped handlers
ctx-seen	Dedup context stages
resolve-complete	Emit trace entries, accumulate paths
get-path-set	Return accumulated path set for includeIf guards
<arg-name>	constantHandler resumes with context value

Compatibility shims

The type system operates at declaration time and cannot be gated on config.den.fxPipeline without circular evaluation:

• aspect-chain in constantHandler — provider functions from providerFnType.merge create { class, aspect-chain } functors
• options.nix uses legacy ctxApply — config.resolved can't access config.den without circularity
• Legacy adapter tests run with fxPipeline = false

Also in this PR

• refactor: replace _. alias with provides. across all aspect/output/template modules
• feat: ci-fast recipe using nix-eval-jobs for parallel test eval
• Test restructuring: batteries/, context/, perf/, home-manager/ subdirs flattened to features/
• Regression reproductions for issues BUG: attempts to use host as conditional of lib.optionalAttrs in sub … #413, BUG: static sub-aspect nixos config silently dropped when included from parametric parent #423, fix: applyDeep skips static subs that don't consume parametric ctx #426, fix: don't coerce factory-function aspects #437
• Consolidated design spec at docs/design/fx-pipeline-spec.md

Test plan

• nix develop -c just ci "" — 488/488 pass
• nix develop -c just ci-fast — parallel eval, 488/488 pass
• nix flake check — templates, packages, devShells all pass
• Checkmate system-agnostic tests — 15/15 pass
• Legacy tests with fxPipeline = false verify backward compatibility

[vic]

Awesome work, @sini 🚀