feat: add CEL as a PDP backend

[araujof]

Summary

Adds support for CEL (Common Expression Language) as a policy decision backend in APL. Authors can write inline boolean predicates over the common attribute vocabulary, evaluated through the existing PdpResolver seam alongside Cedar, OPA, and AuthZen. No changes to APL evaluation semantics.

Usage

global:
  apl:
    pdp:
      - kind: cel
routes:
  - tool: get_document
    apl:
      policy:
        - cel:
            expr: |
              subject.id in ["alice", "bob"]
              && delegation.depth <= 2
              && !("compensation" in session.labels)
          on_deny:
            - deny

Changes

New crate apl-pdp-cel (mirrors apl-pdp-cedar-direct):

• resolver.rs: CelResolver implements PdpResolver. Holds a compile-once cache keyed by expression source, so each distinct expr compiles a single time. true allows, false denies. Compile errors, undeclared-variable references, and non-boolean results are fail-closed (deny), with an on_error: allow config override.
• activation.rs: maps APL's flat dotted-key AttributeBag (subject.id, delegation.depth, session.labels) into nested CEL maps so authors use natural field selection. StringSet maps to a CEL list (enables in and comprehension macros). Namespace wins on leaf/prefix collisions. Author-supplied cel: args (resource, context) surface as variables, with bag values taking precedence.
• factory.rs: CelPdpFactory (kind = "cel") lets the apl-cpex visitor build the resolver from unified-config YAML.

apl-core:

• step.rs: adds first-class PdpDialect::Cel and from_key("cel").
• parser.rs: recognizes the cel: step key with on_allow / on_deny reactions.

Workspace: apl-pdp-cel added to members and default-members.

Crate

cel = "0.13" (clarkmcc, MIT). Sync evaluation, comprehension macros (has, all, exists, map, filter), custom functions. No static type checker, so validation is compile-time parse plus fail-closed runtime.

Tests

• 16 unit tests: bag-to-activation mapping, scalar and set conversion, in / has() / exists() macros, compile-cache reuse, fail-closed and on_error: allow paths, config parsing.
• 2 integration tests: drive a real PluginManager from YAML (kind: cel) through visitor, factory, resolver, and the allow/deny split, proving zero-glue operator wiring.
• apl-core: 216 tests pass (dialect serde, cel: parse).
• cargo build --workspace clean; apl-pdp-cel clippy-clean.

Notes

In the future, we could explore inline cel("...") predicates inside rule conditions (when: / require / redact), reusing the same activation mapping.