refactor: drop stale L1/L2/L3/L4/L5 layer-numbering jargon

[zzylol]

What

docs/pre-asap-ir.md / docs/post-asap-ir.md don't define an L1–L5
layer numbering — #211 explicitly removed it from the docs as
undefined jargon, in favor of the terms the docs actually define:
"pre-ASAP IR" / "post-ASAP IR". The source code still had it all over,
both as real type names and as prose shorthand — surfaced during
#214's review ("what is L3Scalar?" → "does L2 refer to
pre-asap-ir?"). This drops it from the code too.

Type renames (mechanical, no behavior change)

• L2QueryExpr → UnresolvedQueryExpr (QueryExpr<ColumnRef>, the
  front-end-emitted, name-based tree)
• L3QueryExpr → ResolvedQueryExpr (QueryExpr<ColumnId>, the
  positional, resolved tree)
• L4Node → SummaryNode, L4Schema → SummarySchema,
  L4DataType → SummaryDataType, L4Field → SummaryField (all
  four already sat next to SummaryExpr/SummaryKind/SummaryParams
  in post_asap — this makes them consistent with their own module's
  naming instead of a leftover number)

The two front ends' local L2 construction alias
(UnresolvedQueryExpr as L2) becomes Unresolved. A few incidentally
adjacent stale names got the same cleanup while touching their call
sites: df_expr_to_l2 → df_expr_to_unresolved, scalar_value_to_l3
→ scalar_value_to_asap, arrow_to_l3/l3_to_arrow →
arrow_to_dtype/dtype_to_arrow, matcher_to_l3expr →
matcher_to_compare, plus local variables named l2/l3/l4
(tree, resolved, bound, logical_leaf, …) and two test-local
as L2/as L3 aliases (dropped — the bare names already read fine at
those call sites).

Two real name collisions surfaced while doing this: sql/mod.rs and
sql/types.rs both already import datafusion::common::ScalarValue
unqualified — aliased that import to DfScalarValue in both (matching
the DfColumn/ArrowDataType-style convention those files already
use), rather than picking a different name for the asap-types type.

Prose

The much larger remainder was documentation: module docs, doc
comments, and // explanatory comments across nearly every crate
describing a step as "L1 parse", "the L2 tree", "an L4 concern", etc.
Replaced with either the descriptive phrase already used elsewhere in
this repo ("pre-ASAP" / "post-ASAP" / "canonical" / "unresolved" /
"resolved" / "a deployment's own physical stage" for the L5 placeholder
that was never modeled in this crate to begin with) or, where the
sentence just meant "the canonical tree", dropped the label entirely.
A few doubly-stale spots got fixed at the same time since they were in
the exact text being rewritten: bind.rs's / show_post_asap_ir.rs's
references to the deleted asap-ir/asap-lower crate names, and
devtools/src/lib.rs's reference to a "shared L2→L3 converter" that
issue #179 removed (each front end now calls resolve_root directly).

Left alone: boundary.rs's "L2-norm"/"L1-norm" (real math, Count-
Sketch's error bound), and two literal citations of an external design
doc's own section titles ("§6 ... L3 edge", "§'L4 — sketch algebra'")
in schema.rs/l4_binding.rs — that's the other doc's own
numbering, not this crate's.

Verification

cargo build / clippy --all-targets --all-features --locked -- -D warnings / fmt --all -- --check / test --workspace --locked /
doc --workspace --no-deps all clean. Same test counts as before, all
green. cargo doc warnings are one fewer than baseline (fixed the
asap_ir link along the way) — no new warnings introduced. Pure
rename — no behavior change.

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