wb | workbench cleanup, static Make targets, advance genesis

[fmaste]

Removes dead UTxO-HD/RTS knobs and their profiles, replaces the generated Make targets with explicit static rules, and advances the epoch-300 profiles to Voltaire while keeping their 64k block size.

• wb | remove lmdb support with its profiles: drop the LMDB UTxO-HD backend (P.lmdb, Node.utxo_lmdb, the V1LMDB ledger config) and its 4 profiles (ci-bench-lmdb, value-volt-lmdb-nomadperf, value-volt-lmdb-cgmem-nomadperf, plutus-volt-lmdb-nomadperf).
• wb | remove gc parallel and load balance support and its profiles: drop the -qg1/-qb1 RTS knobs (rtsGcParallel/rtsGcLoadBalance) and their 4 rtsqg1 profiles.
• wb | remove RTS profiling from profile definitions, shell parameter only — drop rtsHeapProf and the 3 6-dense-*-rtsprof profiles; the 6-dense-*-rtsprof targets now inject -hT -l via WB_PROFILING at the Make level (a non-profiled build), instead of baking RTS profiling into the profile.
• wb | rewrite Make shell targets as explicit static rules, drop jq: the per-profile shell targets were generated by a proftgt macro built out of eval/call/foreach (meta-programming that produced the targets dynamically: hard to read and impossible to grep). Replace it with one explicit, literal, greppable static-pattern-rule stanza per variant (-nix, -prof, -auto, -autostay, -nomadexec, cloud, ...), each flag emitted as an explicit ARG_*_YES/NO fragment. Also move the workbench Make code under nix/workbench/ (lib.mk, profiles.mk) and drop jq plus the nix build && cat result dance from list-profiles/show-profile/ps.
• wb | move all profiles using genesis epoch 300 to voltaire keeping 64k block size: switch every genesisVariant300 profile to the new genesisVariantVoltaire64k (epoch 537 + the blocksize64k overlay), advancing the genesis while preserving the 64k maxBlockBodySize those profiles already had