feat(core): add workspace skeleton and the pure decision engine

Establishes the cargo workspace and the shared `blocker-core` crate. The
engine is deliberately written before any platform code, because CLAUDE.md
section 2 makes it the single source of decision truth and M1 acceptance
requires it to be pure.

**Workspace** — five crates plus a test crate, zero external dependencies.
Section 3 requires every crate to justify its binary-size cost, so M0 adds
none. [profile.release] is workspace-only (cargo rejects it in members) and
carries the section 5 settings that make the 5 MB stripped budget reachable.

**Purity contract** — `evaluate(&EngineInput) -> Decision` reads no clock, no
disk and no OS API; everything arrives in the input. `tests/purity.rs`
enforces this mechanically by asserting blocker-core has zero dependencies, so
a stray I/O reach-in fails the build rather than silently making the golden
tables useless as a parity contract.

Decisions worth calling out:

- **Monotonic time is a separate type from wall time.** Section 1.5.3 requires
  a system date change never to shorten a lock. Keeping them apart makes that
  structural rather than a rule to remember.
- **`start > end` means overnight.** No `crosses_midnight` flag: the comparison
  already encodes it and a second field is a second way to be wrong. The
  weekday for a window that started yesterday evening is yesterday's, which
  `overnight_window_survives_past_midnight` pins down.
- **`next_wakeup` is armed over every non-paused schedule, not only active
  ones.** Filtering to active conditions first would let the daemon sleep
  straight through the moment a schedule is due to start.
- **Allowlist needs the installed-app inventory.** `EngineInput` carries
  `installed_apps`; an empty slice fails open rather than guessing.
- **Wall-clock correction extends, never shortens, and clamps to the 24h cap.**
  Extending unbounded would satisfy anti-tamper and violate section 1.5.4.
- **`cast_possible_truncation` is denied workspace-wide.** Truncating a
  millisecond counter would corrupt a lock timer.

Verified: 22 tests pass, `cargo fmt --check` clean, `cargo clippy -D warnings`
clean, stripped release daemon 0.279 MB against the 5 MB budget.

Co-Authored-By: Claude Code <noreply@anthropic.com>

rustfmt.toml

@@ -1,12 +1,23 @@
 edition = "2021"
 max_width = 100
 
-# Imports are grouped std / external / crate, and sorted within each group.
-imports_granularity = "Crate"
-group_imports = "StdExternalCrate"
+# NOTE ON UNSTABLE OPTIONS
+#
+# `imports_granularity`, `group_imports` and `derive_debug` are nightly-only
+# rustfmt features. rust-toolchain.toml pins stable, so listing them here emits
+# a warning per crate per file and buys nothing. They are intentionally absent
+# rather than commented out: an unrecognised key is a warning, a commented key
+# is a trap for whoever re-enables nightly.
+#
+# If this project ever moves to nightly rustfmt, add:
+#   imports_granularity = "Crate"
+#   group_imports = "StdExternalCrate"
+#   derive_debug = false
+# as a separate commit, so the diff is auditable.
 
-# We write a lot of match arms over enums and error plumbing. Keep them tight.
+# Trailing comma on match arms adds vertical noise in the evaluator, which is
+# almost entirely match expressions.
 match_block_trailing_comma = false
 
-# Derive Debug on virtually everything — engine output is logged in the field.
-derive_debug = false
+# Use small heuristics instead of the unstable ones.
+use_small_heuristics = "Max"