Record the 22 September forum re-sweep, and clone icepack2 at a pin

[dlilien]

What

Section 8 of antarctica/FORWARD_RUN_READINESS.md: a fourth, read-only pass over the ISMIP7 board on 22 September, one day after section 7. The board holds 50 threads. Forum threads 22, 30, 48, 49 and the new 50 were read in full through the GitHub API; the Greenland thread 47 was skipped. Four files: the readiness doc, one sentence of the submission README, the regenerated NOW.md, and one stale comment in icepack2_tools/forcing.py. No behaviour change.

What moved since the 21st

item	state on 22 September	bears on
isschecker 0.5.1	flux bounds tightened to [-10, 0] and [-10, 10] kg m-2 s-1, the six tend* totals to +/-1e9 kg s-1; experiment table unchanged	#12, #17, #18
forum thread 50, new	the three fluxes are a mass change per unit horizontal cell area, which is how the writer books them	#17
forum thread 49	2300 will not be added to the ocean; the atmosphere group padded 2300 with the 2290-2299 mean; a group asks whether to end in 2299, no organiser has replied	#78, #41
forum thread 30	PISM: the mask everywhere doubles the contribution (1.25 to 2.48 m), at the margins only it has hardly any effect; MRI-ESM2-0 barely responds either way	#10
forum threads 22, 48, 17, 37	unmoved	#17, #11, #18, #16

Verified on Quartz, read-only

The CESM2-WACCM SDBN1-8000m v2 and dEBM2-8000m v1 directories hold 286 years, ending in _2300.nc; the ocean v3 directories end in 2291-2299.nc. Read with h5py, the 2300 acabf, acabf-anomaly and tas match the 2290-2299 mean to 2e-7 relative for both ssp126 and ssp585, against 3e-2 to 5e-1 for 2299 alone. So a run to 2300 reads a padded decadal mean for SMB and a held 2299 for the ocean, and the README's SMB sentence, which claimed the hold for both, is corrected here. Section 1's "the 2300 atmosphere files were removed" is corrected too. The stale comment on the atmosphere bridge in icepack2_tools/forcing.py is corrected in the same terms: that branch fires only when a year is missing, so it no longer fires for the CESM2-WACCM atmosphere and does still cover an older mirror copy. The upstream question is left to #78.

Checked, not changed

• Our gridded fluxes are annual means on the native cells, one to two orders of magnitude inside 0.5.1's new bounds: removing 1000 m of ice in a year is -0.029 kg m-2 s-1, and ligroundf for u = 4 km/yr, H = 1000 m into a 500 m cell is 0.23.
• 0.5.1 still range-checks only non-scalar variables, so the new tend* bounds bite nothing today. A year in which the mask removes a Ross-sized shelf would put tendlicalvf near -5e9 kg s-1, past the new bound, if a later release checks scalars.
• The bundled icepack2_tools/ismip7_variable_request.csv is still 0.5.0's. Bumping it belongs to output: re-run isschecker at 0.5.0 and record the version in the README #12, which this PR does not touch beyond a comment.
• ismip7-antarctic-ocean-forcing, ismip7-scalar-processing and ismip/documentation have no commit or issue since the 20th.

Not done

• Nothing was posted to the forum or to the checker. No issue is closed by the sweep itself; solver: the inversion depends on uncommitted icepack2 edits that exist on one workstation #46 closes with this merge, see below.
• isschecker was read at its tag and never run; no mirror listing was taken.
• The Sites field of the new board card for forcing: CESM2-WACCM 2300 reads a padded 2290-2299 atmosphere mean over a held 2299 ocean, and upstream has not said whether runs end in 2299 #78 could not be set: it is a multi-select, which this API surface neither reads nor writes. Owner is upstream and Status Blocked.

Also here: icepack2 comes from a pin, not an rsync (closes #46)

icepack/icepack2#3 is open and is not expected to merge before the deadline, so this stops waiting for it and pins to its branch instead. The head of hoffmaao/icepack2:fix/firedrake-2026-geometric-dimension, e0a46c9, is icepack2 main (40e848b) plus the two files, confirmed by comparing the two through the API, so a clone of that commit is exactly what the workstation and Rice were running as uncommitted edits.

• install_deps.sh names the remote, the branch and the SHA in three overridable variables, clones them when $SRC/icepack2 is absent, and checks out the pin when an existing clone sits on anything else. --check-icepack2 does that and nothing else, so a site can print what it runs without touching its venv.
• A tree that is not a clone, or that carries local changes, is reported and left alone. At a site still holding the rsynced copy the message says to move it aside: discarding it silently could throw away the only copy of whatever else is in it.
• icepack, tlm_adjoint and icepack_tools are still rsynced, icepack_tools because it has no remote at all.
• tests/test_icepack2_pin.py, six cases against a local repository through the overrides so that nothing reaches the network: clone when absent, move an existing clone onto the pin, refuse and preserve a dirty tree, refuse and preserve an rsynced copy, error on a commit the remote lacks, and pin the three defaults so that following the fix onto main is a deliberate edit.
• batch_runners/readme.md carries all of it, including which sites held the edits on 22 September.

Two annotations pointing at issue #66 are retargeted, since it closed while this branch was open: the re-measure the readiness rows owe belongs to #30.

Full unit suite on the Mac after the pin: 430 passed, 6 skipped, 0 failed (424 before its tests).

Closes #46

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