Deduplicate the four near-identical SRC sweep implementations #17

Problem

apply.py and compress.py contain four implementations —
_src_mpo_mps, _src_mpo_mpo, _src_mpo, _src_mps — that are structurally
identical. Each one is roughly 80 lines with the same five phases:

1. read problem dimensions and log,
2. move the input arrays to the device,
3. left-to-right sweep accumulating the sketched C tensors,
4. right-to-left sweep building the isometries via truncated_qr,
5. handle the first site, convert back to host, wrap in a quimb object.

The only genuine differences are the opt_einsum subscript strings and the
reshape/transpose bookkeeping around them.

On top of that, the module preamble is duplicated verbatim between the two
files: the identical LOG_LTR, LOG_RTL, LOG_TIME and LOG_WARN_SMALL
constants, the same imports, and the same setup_logging() / logger block.

The cost is real and already visible in the history: the terminal-bond
truncation bug had to be fixed in four places, and the cutoff feature had to
be threaded through four times. Each new primitive or fix multiplies by four.

Proposed fix

Factor out the shared skeleton without obscuring the algorithm — the einsum
subscripts should stay visible and readable, because they are the algorithm.

• Move the duplicated LOG_* constants, imports and logger setup into a
  shared private module (src_method/_common.py or the existing utils).
• Extract the parts that are genuinely identical: device transfer of the input
  arrays, the timing/logging wrapper around each sweep, the progressive
  C[j - 1] = None memory release, and the final host conversion plus quimb
  wrapping.
• Express each primitive as its set of contraction subscripts plus the small
  reshape/transpose rules, driven through the common sweep driver. A small
  dataclass or a module-level table of subscripts per primitive keeps this
  declarative and greppable.

Explicit non-goal: do not collapse the four primitives into one clever
generic einsum builder. That would trade a duplication problem for an
unreadable one. The aim is one sweep driver plus four short, obvious
specifications.

Acceptance criteria

• The duplicated LOG_* constants and logger setup exist in exactly one place.
• The left-to-right and right-to-left sweep scaffolding is written once.
• Each primitive's contraction subscripts remain plainly readable.
• No behaviour change: the existing test suite passes untouched.
• No measurable performance regression.