[zayenz]

Allow BoolExpr e = false; and BoolExpr e = true; with a dedicated constant node storing a bool. Materialization returns a fixed Boolean variable; asserting true does nothing and asserting false fails the space. Constants use the existing negation and connective processing.

Keep default-constructed expressions as empty accumulators. Restrict implicit construction to actual bool values to preserve integer and pointer overload resolution, and retain the existing integer result of expr(home, true).

Fixes #145.

Validation:

• Boolean-expression suite: 2,668 cases passed, five iterations and five threads. Includes negated constants in flattened AND/OR, equivalence, implication, assertions, and empty-expression handling.
• Integrity suite: 490 cases passed.
• C++17 downstream compile/link/runtime and overload-compatibility probes passed against the shared libraries.
• Repository tidy and diff checks passed. Native Windows shared-library coverage runs in the existing CI matrix.
• The documentation target has a pre-existing missing statistics marker in test/package/allocator/boundary.hh; that cleanup is already on feature/6.5.0-cleanups.