TRD geometry simplification

[sawenzel]

This is part of the geometry simplification series. TRD is the largest contributor of logical volumes in the ALICE geometry, and almost all of them are duplicates. Gsposp creates a new TGeoVolume on every call, so two parametrised placements per MCM with compile-time-constant parameters produced 7296 identical tubes per supermodule, each carrying a run-time-shaped water core. The baseline counts confirm it: UTCP and UTCH together are 15504 of the 17678 volumes and describe 7 distinct shapes.

The first six commits share those volumes, write out the dimensions that the Geant3 -1 convention resolved at run time, and build twelve chamber shapes instead of thirty. Each was verified separately against dev:

commit	volumes	nodes	matrices	geometry file	hits vs dev
dev	17678	23006	15011	1007 kB	—
1 · shared MCM pipe stub	3088	15711	15011	749 kB	identical
2 · per-layer row pipes, power lines	1738	15261	15011	720 kB	identical
3 · explicit chamber dimensions	1558	15261	15011	693 kB	identical
4 · integer hit lookup	1558	15261	15011	693 kB	identical
5 · twelve chamber shapes	1072	7233	7109	389 kB	identical
6 · per-layer front gas pipes	1002	7198	7109	387 kB	identical
7 · sector-17 steel wall (fix)	1002	7198	7109	387 kB	98/912639 move

Identical means all 11 fields of 912639 TRDHit from 200 seeded muon events, plus 200k sampled points agreeing on material, density and medium in the ideal and the aligned geometry. The overlap count stays at 183 and every alignable ROC entry still resolves.

The last commit is a fix rather than a cleanup: the −x gas pipe of sector 17 placed the Xe core instead of the steel tube, so it has had no wall since the AliRoot port, and TRD_Fe goes from 0 % to 4.7 % in a 1.6 cm sphere around it. It moves 98 hits, which is why it is kept separate from the six neutral commits.