BE-457: HashQL: MIR execution pipeline extensions for postgres compilation

[indietyp]

🌟 What is the purpose of this PR?

Prepares the MIR execution pipeline for consumption by the postgres compiler. The execution analysis now produces a complete IslandGraph (not just a flat island list), the placement solver and island placement accept external allocators, and the traversal system gains the TraversalPathBitMap and as_symbol() APIs that the SQL generator needs. Also adds a backend switch cost to the terminator placement so cross-backend transitions are no longer free.

🔍 What does this change?

Execution analysis (pass/execution/mod.rs):

• run() becomes run_in() with an explicit allocator parameter, returning ExecutionAnalysisResidual (assignment + island graph). The island graph is now constructed as part of the analysis rather than left to the caller.
• Adds run_all_in() which runs the execution analysis over all graph-read bodies in a DefIdSlice.

Traversal system (traversal/mod.rs, traversal/entity.rs):

• TraversalPathBitMap: per-vertex-type collection of TraversalPathBitSets with pointwise lattice operations. The postgres compiler uses this to track which paths each island accesses across all vertex types.
• TraversalPath::as_symbol(): returns a static symbol for each path variant, used as SQL column aliases so the interpreter can locate result columns by name.
• TraversalPathBitSet::vertex(): returns the vertex type for a bitset.
• EntityPath::as_symbol() and EntityPath::column_name(): per-path SQL identifiers.

Terminator placement (terminator_placement/mod.rs):

• TransMatrix gains AddAssign for element-wise saturating addition.
• Adds backend_switch_cost() which encodes a fixed overhead for cross-backend transitions (Postgres to Interpreter: 8, Interpreter to Embedding: 4, etc.). Previously cross-backend transitions had zero inherent cost, so empty blocks were arbitrarily assigned to the interpreter even when staying on postgres was free.

Placement solver (placement/solve/):

• PlacementSolver::run() becomes run_in() with allocator parameter.
• CSP solver internal allocations use the provided allocator.

Island graph (island/graph/mod.rs):

• IslandGraph::new_in() now takes an allocator for its output storage.

Pretty printer (pretty/text.rs):

• TextFormatAnnotations gains annotate_basic_block() and BasicBlockAnnotation associated type, plus a blanket impl for &mut T.

Builder (builder/rvalue.rs):

• Adds RValueBuilder::opaque_entity_uuid() convenience constructor for the common pattern of extracting an entity's UUID.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

⚠️ Known issues

The backend_switch_cost() values (8, 4, etc.) are hand-tuned heuristics. They correctly prevent the solver from arbitrarily switching backends for empty blocks, but a proper cost model would derive these from measured overhead.

🛡 What tests cover this?

• Updated execution pass end-to-end tests (execution/tests.rs) including entity_uuid_equality, mixed_postgres_embedding_interpreter, projection_and_apply_splits
• Updated statement placement tests for interpret and postgres backends (eq_opaque_entity_uuid)
• Updated terminator placement and island tests
• Placement solver tests updated for allocator API

❓ How to test this?

cargo nextest run -p hashql-mir