Preserve size=1 vector parameters and add a prior_draw_mode contract

[vhaasteren]

Summary

• Fix PTA.map_params to preserve a size=1 parameter as a length-1 array
  instead of collapsing it to a bare scalar (n = p.size if p.size else 1
  conflated size=None and size=1). A one-dimensional vector parameter
  mapped to a scalar breaks downstream linear algebra (e.g. C @ x on a
  1x1 whitening matrix).
• Add Parameter.prior_draw_mode (default "component", class-level
  attribute so every existing Parameter subclass inherits it unchanged)
  and UserParameter(..., prior_draw_mode="component"|"joint"), so a
  correlated vector parameter can declare that prior-draw proposals must
  replace it as a whole block rather than one component at a time.
• Defensive consistency fix: param_names (on Signal, SignalCollection,
  and PTA) now checks p.size is not None instead of the previous truthy
  if p.size: check, for the same size=None vs size=1 distinction
  map_params needed. (param_names already produced the correct indexed
  name for size=1 today; this just makes the two checks consistent.)

Why

Part of implementing nltiming's feature_nltiming_sampling_api.md: a
full-whitening nonlinear-timing block is a correlated joint vector Enterprise
parameter, and when only one timing parameter is sampled that block has
size=1. Without the map_params fix it silently collapsed to a scalar;
without prior_draw_mode, a generic prior-draw proposal (e.g.
enterprise_extensions.JumpProposal.draw_from_prior) has no way to know it
must replace such a block atomically rather than one entry at a time, which
is mathematically invalid for a non-diagonal (correlated) prior.

prior_draw_mode is deliberately generic — no NLT-specific attribute or name
inspection — so it's reusable by any future correlated vector parameter, not
just this one.

Test plan

• pytest tests/test_parameter.py tests/test_vector_parameter.py — includes
  new TestPriorDrawMode and TestMapParamsVectorShape coverage (25 tests,
  all pass).
• black --check / flake8 clean on the changed files.
• Ran the rest of the suite file-by-file (test_likelihood.py,
  test_selections.py, test_set_parameter.py, test_utils.py, etc.) — all
  pass.

Depends on / paired with

Companion PRs on enterprise_extensions (honor prior_draw_mode in
JumpProposal) and nltiming (Enterprise-native timing parameters that use
this contract).

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.85%. Comparing base (5b3d80c) to head (544a737).
⚠️ Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
enterprise/signals/signal_base.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #455      +/-   ##
==========================================
+ Coverage   72.71%   72.85%   +0.14%     
==========================================
  Files          13       13              
  Lines        3478     3489      +11     
==========================================
+ Hits         2529     2542      +13     
+ Misses        949      947       -2     

Files with missing lines	Coverage Δ
enterprise/signals/parameter.py	91.83% <100.00%> (+0.19%)	⬆️
enterprise/signals/signal_base.py	90.92% <91.66%> (+0.06%)	⬆️

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