Bump version to 9.6.166 for Plinth 1.66

The third component of the version now encodes the Plinth version the
compiler targets (1.66 -> 166) rather than being an opaque marker, so that
users can tell which plutus-tx a given uplc-ghc has to be used with. Nothing
else in the version conveyed that, and the coupling is strict: uplc-ghc only
compiles Plinth code against the plutus-tx it was built against, which
plinth-test.cabal pins to ^>=1.66.0.0.

Re-releases of the same Plinth version are distinguished by the fourth
component (9.6.166.X for RELEASE=YES, 9.6.166.DATE otherwise), so the
trailing release counter that the old 1501 carried in the third component is
dropped.

Also document the scheme in configure.ac, which recorded neither the meaning
of 1501 nor that the minor is written with two digits -- a future Plinth 1.7
must map to 170, not 17, or the version silently goes backwards.

Note that 166 is numerically below 1501. Nothing was ever published as 1501
(there are no tags yet), so no public version regresses; doing this before
the first release keeps the numbering coherent from v1 onward.

Since the version is part of the package unit-ids and keys cabal's store
directory (ghc-9.6.166-plinth), this invalidates existing _build trees and
any dependencies previously built against 9.6.1501.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: hsyl20

configure.ac

@@ -13,16 +13,33 @@ dnl
 # see what flags are available. (Better yet, read the documentation!)
 #
 
-AC_INIT([The Glorious Glasgow Haskell Compilation System], [9.6.1501], [glasgow-haskell-bugs@haskell.org], [ghc-AC_PACKAGE_VERSION])
+AC_INIT([The Glorious Glasgow Haskell Compilation System], [9.6.166], [glasgow-haskell-bugs@haskell.org], [ghc-AC_PACKAGE_VERSION])
     # Version on master must be X.Y (not X.Y.Z) for ProjectVersionMunged variable
     # to be useful (cf #19058). However, the version must have three components
     # (X.Y.Z) on stable branches (e.g. ghc-9.2) to ensure that pre-releases are
     # versioned correctly.
 
-    # Note for Plinth release: Up to four version components are supported and,
-    # we want a distinct version from upstream GHC. Therefore we use:
-    #  - 9.6.1501.DATE for RELEASE=NO (version in this file is [9.6.1501])
-    #  - 9.6.1501.X for RELEASE=YES (add X to version: [9.6.1501.X])
+    # Note for Plinth release: up to four version components are supported (the
+    # maximum tools accept) and we want a version distinct from upstream GHC's.
+    #
+    # The third component encodes the Plinth version this compiler targets, using
+    # a two-digit minor: Plinth 1.66 -> 166 (a future 1.7 is 170, not 17). Bump it
+    # whenever the plutus submodule is updated, and keep the plutus-tx bound in
+    # plinth/test/plinth-test.cabal in step: uplc-ghc only compiles Plinth code
+    # against the plutus-tx it was built with, and nothing else in the version
+    # would tell a user which one that is.
+    #
+    # The fourth component distinguishes builds of the same Plinth version:
+    #  - 9.6.166.DATE for RELEASE=NO (version in this file is [9.6.166])
+    #  - 9.6.166.X for RELEASE=YES, X counting re-releases from 1
+    #
+    # 166 is numerically below the previous 1501, which encoded a release counter
+    # in this component; nothing was ever published as 1501, so no public version
+    # went backwards.
+    #
+    # Note that this version also keys cabal's store directory
+    # (~/.cabal/store/ghc-9.6.166-plinth), so each Plinth version gets its own
+    # store and cannot reuse ABI-incompatible artifacts built by another.
     #
     # In GHC 9.8+ we don't need to modify the version anymore, just the
     # "Project Unit Id" from ghc --info