purge Avalanche-lineage keywords (one pure modern version)

Neutralise the promotional surface around legacy upstream upgrade names
(Apricot / Banff / Cortina / Durango / Etna / Fortuna / Granite) while
preserving the on-disk Config schema and the IsXxxActivated() predicate
surface — those field names and methods are load-bearing for every
upstream-derived block parser, codec, and tx executor in the tree, and
Lux activates every gate at chain birth so they are inert in production.

Changes:

- upgrade/upgrade.go: add struct-level comment on Config explaining that
  every gate is active from InitiallyActiveTime (Dec 5 2020) so the
  IsXxxActivated() predicates are inert compatibility surfaces; Lux-
  native gating belongs in the ChainSecurityProfile.
- upgrade/upgradetest/fork.go: add package-level comment marking the
  Fork enum as a compat surface for upstream-derived test fixtures.
- vms/platformvm/docs/{block_formation_logic,chain_time_update}.md:
  rewrite from pre/post-upgrade narrative into single-shape
  documentation of the only model that runs on Lux.
- vms/platformvm/docs/validators_versioning.md, indexer/service.md,
  config/config.md, vms/platformvm/warp/README.md: drop pre/post-fork
  references from prose, leave the protocol description intact.
- vms/proposervm/proposer/windower.go, pre_fork_block.go,
  vms/proposervm/block_test.go, vms/platformvm/warp/validator.go,
  vms/platformvm/txs/executor/proposal_tx_executor.go and a handful of
  test helpers: rewrite descriptive comments and the four
  "Banff fork time" error messages to refer to the Config field name
  (upgrade.Config.BanffTime / DurangoTime / GraniteTime) instead of
  the upstream brand label.
- tests/granite_integration_test.go -> tests/lp181_integration_test.go:
  rename file and Test/Benchmark functions to LP-181-relative names;
  field accesses (GraniteTime, GraniteEpochDuration,
  IsGraniteActivated) preserved because they are the on-struct names.
- network/network.go, network/peer/peer.go, chains/manager.go,
  scripts/tests.upgrade.sh, .github/labels.yml: scrub stray
  upstream-brand mentions from comments / labels.

What is intentionally preserved (compat shim policy, same logic as the
geth-config preservation rule for ~/work/lux/coreth /geth /genesis):

- upgrade.Config field names (ApricotPhaseNTime, BanffTime, ..., GraniteTime)
  and IsXxxActivated() predicates — JSON tags pin the on-disk schema.
- upgradetest.Fork enum values (NoUpgrades..Granite) — referenced by
  hundreds of upstream-derived test fixtures.
- Block-type identifiers (BanffStandardBlock, ApricotProposalBlock, ...)
  — wire-format-load-bearing.
- chainadapter/* entries naming "Avalanche" as a cross-chain integration
  target alongside Bitcoin/Ethereum/Solana/Polygon — these name external
  networks Lux bridges to, not Lux's own upgrade lineage.
- RELEASES.md historical release notes — author/PR credit data from
  upstream history is not promotional brand text.

Build verified: GOWORK=off go build ./... -> exit 0.

indexer/service.md

@@ -47,7 +47,7 @@ Each chain has one or more index. To see if a C-Chain block is accepted, for exa
 ```
 
 <Callout type="warn">
-To ensure historical data can be accessed, the `/ext/index/X/vtx` is still accessible, even though it is no longer populated with chain data since the Cortina activation. If you are using `V1.10.0` or higher, you need to migrate to using the `/ext/index/X/block` endpoint.
+To ensure historical data can be accessed, the `/ext/index/X/vtx` is still accessible, even though it is no longer populated with chain data since the X-Chain block linearization landed. If you are using `V1.10.0` or higher, you need to migrate to using the `/ext/index/X/block` endpoint.
 </Callout>
 
 ## Methods