Posterior predictive checks for trained NPE posteriors

[maho3]

Adds an end-to-end posterior predictive check (PPC): draw parameters from
q(theta | x_obs) at a single test point, resimulate each draw through the
identical forward chain, and compare the resulting data vectors against the
observed one.

This replaces the importance-sampling approach in cmass/infer/resim.py, which
reweights existing simulations rather than generating new ones and collapses in
high dimension: on abacuslike/fastpm_charm6_comphod (17 parameters) it gave
ESS = 3 out of 19777. Drawing and simulating directly has no such failure mode.
resim.py is left in place and untouched by this PR; the PPC scripts reuse its
loaders and plotting helpers.

Validated on a 100-simulation self-consistent run. Write-up:
ltu-gobig-notes/experiments/2026-08-13_ppc_abacuslike-fastpm_charm6_comphod/.

What's new

All of it lives in a new root-level ppc/ folder, following the power_tests/
precedent (feature folder, unprefixed filenames, a README).

File
ppc/README.md	Stages, how to adapt to another experiment, outputs
ppc/draw.py	Draw theta from the posterior; write the cosmology file, per-draw hydra overrides, manifest, posterior_draws.npz
ppc/slurm_nbody.sh	Stage A: FastPM, array over draws
ppc/run_charm.sh	Stage B: CHARM. Not a SLURM script, run by hand on a GPU machine
ppc/slurm_hod.sh	Stage C: apply_hod + diagnostics.summ, array over draws
ppc/slurm_collect.sh	Stage D: collect + figures
ppc/collect.py	Build x_ppc.npy / theta_ppc.npy, verify every draw, plot bands / corner / logprob

One library change

cmass/bias/rho_to_halo.py: apply_charm_new takes optional charm_yaml and
charm_ckpt, wired to bias.halo.charm_yaml / bias.halo.charm_ckpt. Both
default to the module-level CHARM_YAML / CHARM_CKPT constants, so behaviour
is unchanged when the keys are absent. This exists because reproducing an older
suite requires its CHARM checkpoint, and the path was previously hardcoded: the
PPC target suite is charm6 (charm_joint_best_val_ft15.pth) while HEAD defaults
to charm7 (charm_joint_v19.pth). Running with the wrong one silently swaps the
halo bias model.

Design notes for review

• Parameter injection. Cosmology goes through a generated cosmology file
  indexed by nbody.lhid, HOD through a full bias.hod.theta dict override
  (a dotted override does not attach, since bias.hod.theta defaults to
  None), noise through noise=fixed. bias.hod.seed=1 is kept so
  parse_hod still derives lhid*1e4+1 for galaxy placement as in training;
  the sampled parameters it produces are then fully overwritten by the theta
  dict.
• Independent IC phases. matchIC=0, so gen_white_noise seeds on the
  draw index. Draws are not phase-matched to the observed simulation, which
  they must not be.
• Verification is not optional. ppc/collect.py checks every draw's
  recorded cosmology, HOD and noise parameters in its diagnostics file against
  the drawn values, drops mismatches from x_ppc and theta_ppc together, and
  records the reason in manifest.tsv. It aborts rather than write a
  misaligned array if the block layout does not match the training data, and
  refuses to run if a pca.pkl is present (it must be applied, never refit).
• Appending is append-only. --start N carries the first N draws over
  verbatim from the existing npz and draws only the new ones, so extending a
  campaign cannot perturb draws that have already been simulated.
• Held-out summaries. The bands figure covers every available z-space
  summary, not just the ones the posterior was conditioned on; panel titles
  mark which is which. Observed vectors for held-out summaries are recomputed
  from the observed simulation's own diagnostics, located by matching recorded
  HOD and noise parameters rather than by filename.

Known issue, not fixed here

Training summaries produced before ba2334f (pylians to pypower for
periodic-box P(k)) sit on a uniform dk=0.01 grid, while current runs sit on
pypower's effective-k grid. Both yield 39 bins at kmax=0.4, so shapes match and
nothing errors, but bin centres differ by up to 6.7% at the lowest k (under 2%
in P, confined to the three lowest bins; negligible above k~0.1). Bispectrum
binning is unaffected. Left as-is by decision: the affected summaries are stale
and future runs are pypower throughout. ppc/collect.py documents this and
plots against the PPC k values. Worth knowing before comparing any pre- and
post-ba2334f data vector, not only in the PPC.

Not covered

No Mahalanobis distance or p-value is computed. x_ppc.npy and theta_ppc.npy
are row-aligned and x_obs is in posterior_draws.npz so that a distance
statistic is trivial to add downstream; the conservative band coverage and the
asymmetric predictive distributions for zQk0 / zQk2 argue against applying a
Gaussian statistic blindly.

Before merging

• The whole ppc/ folder is already staged: draw.py, run_charm.sh and
  slurm_nbody.sh as renames out of scripts/ and jobs/ (git detects them at
  92-100% similarity), the rest as new files. Nothing further to git add.
• The branch also carries unrelated work (charm7 integration, MTNG jobs, other
  analysis scripts, job reorganisation into jobs/old/). Scope the merge
  deliberately; the diff against main is far larger than this PR describes.
• Paths in the job scripts are Delta-specific (account bdne-delta-cpu, wdir
  /work/hdd/bdne/maho3/cmass-ili), consistent with the other job scripts in
  jobs/.