CH3CCl3 concentration constraint registered in C++ but no column in SSP CSVs

[shyamperi]

Summary

The C++ source registers a concentration-constraint input
CH3CCl3_constrain for the methyl chloroform (CH3CCl3) halocarbon
component — same shape as every other halocarbon component. But the
SSP CSVs under inst/input/tables/ ship only the CH3CCl3_emissions
column; there is no CH3CCl3_constrain column anywhere in the
shipped tables.

Effect: the constraint input is registered (it appears in init() +
getRegisteredInputs()) but cannot be bound from any default CSV
template. A user-supplied .ini that references
CH3CCl3_constrain=csv:tables/<file>.csv finds no matching column
and silently no-ops the constraint. Same observable symptom as #807,
different cause: there the column was misnamed; here it is absent.

Reproduction

$ git checkout 9631b05f6e139303436c48c9d80ebb8425e88708   # v3.5.0 commit

$ grep -n 'CH3CCl3' inst/include/component_data.hpp
89:#define D_RF_CH3CCl3         D_RF_PREFIX     CH3CCl3_COMPONENT_BASE
121:#define D_RFADJ_CH3CCl3     D_RFADJ_PREFIX  CH3CCl3_COMPONENT_BASE
150:#define D_EMISSIONS_CH3CCl3 CH3CCl3_COMPONENT_BASE EMISSIONS_EXTENSION
178:#define D_CONSTRAINT_CH3CCl3 CH3CCl3_COMPONENT_BASE CONC_CONSTRAINT_EXTENSION
218:#define D_HCRHO_CH3CCl3     D_HCRHO_PREFIX  CH3CCl3_COMPONENT_BASE
248:#define D_HCDELTA_CH3CCl3   D_HCDELTA_PREFIX CH3CCl3_COMPONENT_BASE

# D_CONSTRAINT_CH3CCl3 expands to "CH3CCl3_constrain" — registered.

$ awk -F',' 'NR==6 {for(i=1;i<=NF;i++) if ($i ~ /CH3CCl3/) print $i}' \
    inst/input/tables/ssp245_emiss-constraints_rf.csv | sort
CH3CCl3_emissions
# (CH3CCl3_constrain absent — every other halocarbon has both.)

Compare with a halocarbon that has both columns:

$ awk -F',' 'NR==6 {for(i=1;i<=NF;i++) if ($i ~ /CFC11_/) print $i}' \
    inst/input/tables/ssp245_emiss-constraints_rf.csv | sort
CFC11_constrain
CFC11_emissions

Reproduces in all 9 SSP CSVs (ssp119, ssp126, ssp245, ssp370,
ssp434, ssp460, ssp534-over, ssp585, picontrol).

Possible causes (need maintainer judgement)

1. CSV generator omission. JGCRI/hectordata generates these
   tables; the column list may simply have skipped CH3CCl3. Fix is
   in hectordata: add the CH3CCl3_constrain column to the generator
   alongside CH3CCl3_emissions.
2. Intentional emissions-only treatment. CH3CCl3 has been phased
   out under the Montreal Protocol; its post-2000 atmospheric burden
   is dominated by the residual emissions trajectory rather than any
   constraint scenario. If that's the policy choice, then the C++
   D_CONSTRAINT_CH3CCl3 macro should not be wired through
   registerInput() — leaving it dangling is the actual bug.

Proposed fix

Maintainer's call between two options:

Option A — add the column. Generate CH3CCl3_constrain in
hectordata for every SSP table, with concentration values matching
the modelled trajectory (zero post-phaseout is fine; users who want
a constraint can edit). Matches the symmetry of every other
halocarbon.

Option B — drop the constraint registration. Remove
D_CONSTRAINT_CH3CCl3 from CH3CCl3_halocarbon_component's
registerInput() calls. The registry now reflects only what the CSV
templates actually carry; downstream consumers stop seeing a
dangling input.

We slightly prefer A for consistency with the other 25 halocarbons,
but either is correct.

Impact

• Direct: Users cannot constrain CH3CCl3 concentration via any
  default CSV path. Workaround is the same as HFC245fa concentration constraint column missing 'fa' suffix in SSP CSVs #807: add a
  user-CSV with the column + an .ini binding.