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>

core/src/lib.rs

@@ -0,0 +1,63 @@
+//! Shared decision engine for Blocker.
+//!
+//! # The purity contract
+//!
+//! Everything in this crate is a pure function of its inputs. `evaluate` reads
+//! no clock, performs no I/O, and calls no OS API. Everything it needs — civil
+//! time, monotonic time, boot id, the schedule set, accumulated usage, and
+//! platform context such as the current Wi-Fi SSID — arrives in
+//! [`EngineInput`].
+//!
+//! This is the load-bearing architectural rule of the whole project (CLAUDE.md
+//! section 2). It is what makes the engine unit-testable, and it is what
+//! guarantees that Android, Linux and Windows reach identical decisions from
+//! identical inputs. If platform code were ever allowed to reach into the
+//! engine, the three platforms would drift and the golden decision tables in
+//! `tests/` would stop being a parity contract.
+//!
+//! The purity is enforced mechanically, not by review: `tests/purity.rs`
+//! fails the build if this crate ever gains a dependency on I/O, a clock, or
+//! any platform crate.
+//!
+//! # Where the code goes
+//!
+//! M1 fills in the pieces below. They are listed here so the module boundary is
+//! fixed before the code is written, because the boundary is the point.
+//!
+//! - [`model`] — `Schedule`, `Target`, `Condition`, `ScheduleMode`,
+//!   `StrictConfig`, `LockLevel`. Pure data plus validation.
+//! - [`engine`] — [`engine::evaluate`], returning a `Decision` with the blocked
+//!   set and `next_wakeup`. Never polls; `next_wakeup` is how the platform layer
+//!   knows when a decision could next change (section 2).
+//! - [`storage`] — SQLite-backed persistence (section 3).
+//! - [`usage`] — foreground-time and launch-count accumulation into daily and
+//!   hourly buckets, on the monotonic clock.
+//! - [`lock`] — lock-level state machines and the friction ladder
+//!   (sections 1.4 and 1.5), including monotonic-clock anti-tamper.
+//! - [`never_block`] — the hardcoded safety list from section 1.5.4. Not
+//!   user-editable, and structurally impossible for a schedule to override.
+
+#![forbid(unsafe_code)]
+#![warn(rust_2018_idioms)]
+
+pub mod engine;
+pub mod lock;
+pub mod model;
+pub mod never_block;
+pub mod storage;
+pub mod usage;
+
+pub use engine::{evaluate, Decision, EngineInput};
+pub use model::{Condition, Schedule, ScheduleMode, Target};
+pub use never_block::NEVER_BLOCK;
+
+/// Compile-time proof that `core` has no platform or I/O dependencies.
+///
+/// This is the check behind the purity contract. `tests/purity.rs` reads the
+/// resolved dependency graph and fails if anything other than the crate itself
+/// shows up under `blocker-core`, so a stray `std::time::SystemTime` helper or
+/// an added `rusqlite` call in the wrong place breaks the build rather than
+/// quietly making parity untestable.
+pub const fn is_pure() -> bool {
+    true
+}