Aiur kernel: fix Lean.Syntax.rec (shard 53) + ix name-of

[arthurpaulino]

IxVM Aiur kernel: fix Lean.Syntax.rec (shard 53) aux-recursor gen

ix check --interp bytecode Lean.Syntax.rec failed with assert_eq mismatch: 0 != 1 on the declared-vs-canonical type equality. Three
interlocking bugs in the Aiur block-flattening / recursor-type builder:

• Single-pass flat traversal. build_flat_block scanned originals
  once; nested-aux members (Array Syntax, List Syntax) never had
  their own ctors scanned, so flat had 2 motives when Lean's recursor
  declares 3. Replaced with a queue-based fixed point mirroring
  crates/kernel/src/inductive.rs:build_flat_block:531-599.

• is_rec_field matched on const idx alone. For Lean.Syntax.ident
  the field preresolved : List Preresolved shares the base List
  const idx with the block's List Lean.Syntax aux, so it got a
  spurious motive_2 preresolved IH binder. Match key is now
  (head_idx, spine-arg prefix ≡ member.spec_params) — direct members
  carry spec_params = [] and match on idx alone; auxes require the
  concrete occurrence via new find_flat_member_match +
  spine_prefix_eq + kexpr_struct_eq helpers.

• build_all_minors shrunk flat in-flight. It iterated flat and
  passed the shrinking suffix into build_minor_doms, so later members'
  ctor-field classification was blind to earlier members. Split into a
  thin wrapper build_all_minors + build_all_minors_walk that pins
  the caller's full flat while iteration state shrinks.

Every recursor gen call site (build_minor_at_depth, build_minor_doms,
build_rule_rhs, populate_rules) now threads the full flat alongside
flat_idxs.

Ships pinned FFT cost for Lean.Syntax.rec in the ixvm test suite and
re-pins every stdlib / primitive / prelude cost that shifted from the
codegen refresh. List.rec, Array.rec, Lean.Syntax.rec_1,
Lean.Syntax.rec_2 verified to still pass.

ix name-of: reverse-lookup a Lean.Name from its content address

ix name-of --ixe <path> <hex>: given a 32-byte hex address, scan the
.ixe env's named map and print the fully-qualified Lean.Name of
the const stored at that address. Exits 1 with an error if no named
const matches (unnamed intermediate const or a blob).

Complements ix addr-of (name → address). Primary use: backtrack from
an assertion-failure address logged mid-kernel-execution to the
offending Lean constant.

Test plan

• lake test -- --ignored ixvm passes end-to-end with the new pins.
• ix check --interp bytecode Lean.Syntax.rec exits 0.
• ix check Lean.Syntax.rec (codegen path, after ix codegen) exits 0.
• ix check --interp bytecode List.rec Array.rec Lean.Syntax.rec_1 Lean.Syntax.rec_2 all exit 0.
• ix name-of --ixe <ixe> <hex-of-known-named> prints the expected fully-qualified name; unknown address exits 1.