refactor(resolution): extract build() passes + move Rust FQN tier behind the post_process seam

[coseto6125]

What

Extracts Pass 1.5 (Route extraction), 1.6 (fetch-shape extraction), 1.7
(entry-point scoring), and 1.8 (FunctionMeta collection) out of
GraphBuilder::build() into named module-level functions with narrow
parameters, mirroring the existing pass2_emit_node_edges /
pass2_emit_framework_and_fanout pattern already in this file.

build() drops from ~1252 lines to ~1204 lines and now reads as an
orchestrator calling pass1_5_extract_routes -> pass1_6_fetch_shapes
-> pass1_7_entry_points -> pass1_8_function_metas -> Pass 2 -> Pass 3
-> Pass 4, instead of having pass boundaries that existed only as
comments.

Why

build() was a single 1252-line function; the four Pass-1.x blocks each
had a clean, narrow dependency surface (read local_graphs + some prior
pass's output, write into nodes / string_pool / an owned accumulator)
that made them straightforward to cut without touching control flow.

Scope note: Tier 3.5 (Rust FQN) was NOT moved

The originating review described Tier 3.5 as "~250 lines of Rust-only
crate::a::b::fn FQN resolution inlined directly in build(), bypassing
the post_process seam." That description does not match the code as it
exists on main: the actual FQN resolution machinery already lives
cleanly in crates/ecp-analyzer/src/rust/module_tree.rs
(RustWorkspaceModTree, 1113 lines) and is consumed through
Resolver::with_mod_tree(). What sits inline in build() is ~10 lines
of mod_tree_opt construction plus wiring it into the serial/parallel
Pass 2 resolvers — already about as thin as a call site gets. There is no
250-line inlined block to extract, and no post_process-style seam to
retrofit here without manufacturing indirection that doesn't correspond
to anything in the code. This part of the task is skipped as based on a
stale premise; flagging it explicitly rather than silently dropping it.

Verification (zero behavior change)

• cargo test -p ecp-analyzer --tests: 2512 passed, 0 failed
• cargo test -p egent-code-plexus --tests: 1249 passed, 0 failed
• cargo clippy -p ecp-analyzer --tests: clean
• cargo clippy -p egent-code-plexus --tests: clean
• rustfmt --edition 2021 on the touched file: no changes needed
• Diff reviewed end-to-end: pure code motion, pass order unchanged, no
  dropped statements, no reordered side effects across pass boundaries.

[github-actions]

ecp impact cache (0 symbols) — internal, used by ecp dev pr-analyze

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