refactor(node): W1b-0 — seams/ scaffold, relocate decoupled helper modules (#1285)

[MichaelTaylor3d]

What

W1b-0: first sub-PR of the W1b seam carve (#1285). Creates the seams/ module tree and
relocates the helper modules that have zero impl Node coupling (pure file moves,
no logic change) to validate the seams/ pattern before tackling the tangled god-struct
methods in later sub-PRs.

Sub-PR sequence (W1b, this is #1 of 7)

• W1b-0 (this PR): seams/ scaffold + relocate decoupled helpers — near-zero risk.
• W1b-1: carve seam content (content_serve.rs impl Node methods → ContentServer trait/handle).
• W1b-2: carve seam dig_peer (peer.rs impl Node methods → PeerNetwork trait/handle).
• W1b-3: carve seam chia_peer (chainwatch.rs + AnchoredRootResolver → ChainSource trait/handle).
• W1b-4: carve seam capsule (cache_*/gap_fill/backfill methods → CapsuleStore trait/handle).
• W1b-5: carve seam dig_rpc (RPC dispatch methods in lib.rs → RpcDispatch trait/handle).
• W1b-6: carve seam key_mgmt (identity_seed lifecycle → KeyManager trait/handle); wallet seam
  stays a placeholder per the locked plan (embedded dig-wallet is W5, out of scope here).

Behaviour-preserving throughout — no logic/algorithm change, no seam-crate adoption, no new deps.

Closes nothing yet (tracking #1285).

[MichaelTaylor3d]

PASS — single correctness gate (fresh context).

Verified this is a pure, behaviour-preserving relocation of zero-coupling modules with stable re-exports:

• All 7 moves are byte-identical. git reports similarity index 100% renames for bandwidth.rs + verification_ledger.rs → seams/content/ and net.rs + pex.rs + dht.rs + address_book.rs + session.rs → seams/dig_peer/. No function bodies, tests, or logic changed.
• Zero impl Node coupling confirmed in all 7 relocated files (grep count 0 each) — they define standalone types/fns, correctly the first to carve cleanly.
• Re-exports keep every original path importable. lib.rs drops the old pub mod lines and adds pub use seams::content::{bandwidth, verification_ledger}; + pub use seams::dig_peer::{address_book, dht, net, pex, session};, so crate::net/crate::pex/… (and external dig_node_core::net) resolve unchanged. No caller edits.
• New scaffold only: seams/mod.rs + seams/content/mod.rs + seams/dig_peer/mod.rs, doc-comment modules declaring the moved submodules. No duplicate defs, no dangling refs.
• Versions bumped + consistent: workspace 0.47.1→0.47.2 and dig-node-core 0.13.1→0.13.2 (both patch, correct for a no-behaviour-change refactor).
• 0 unresolved review threads.

Non-gating note (resolved by me): the crate-root /// doc comments previously on pub mod session / pub mod verification_ledger were dropped from lib.rs; their content is preserved in the moved files' own //! docs and summarized in the new seam mod.rs files. No action needed.

Merge-ready.