Issue #35944 | BEFORE | 97cfc07759fed2a67edef22de4370ed74ff40030
Linux x64 | Node v26.12.1 | real source CLI
Fresh isolated HOME / T3 home; service-manager buses disconnected.
No BootService stub; no extracted updater; no test runner.

$ node src/bin.ts service install --base-dir <isolated-home>

BEFORE service.env:
CUSTOM="first
T3CODE_LAN=example
last"

CLI exit: 1
[04:51:42.237] ERROR (#2): BootServicePrerequisiteError: [user-manager-unavailable] Cannot reach the systemd user manager. Run `systemctl --user status` in a login session for the service user. Install your distribution's systemd user-session support if it is missing; do not run T3 with sudo.
    at file://<checkout>/apps/server/src/cloud/bootService.ts:1132:11
    at file://<checkout>/node_modules/.pnpm/effect@4.1.3/node_modules/effect/dist/internal/effect.js:152:23
    at Array.map (<anonymous>)
    at file://<checkout>/node_modules/.pnpm/effect@4.1.3/node_modules/effect/dist/internal/effect.js:149:33
    at file://<checkout>/node_modules/.pnpm/effect@4.1.3/node_modules/effect/dist/Function.js:79:18
    at cloud.boot_service.run_step (file://<checkout>/apps/server/src/cloud/bootService.ts:1124:12)
    at cloud.boot_service.run_step (definition) (file://<checkout>/apps/server/src/cloud/bootService.ts:1025:26)
    at cloud.boot_service.install (file://<checkout>/apps/server/src/cli/service.ts:122:31)
    at cloud.boot_service.install (definition) (file://<checkout>/apps/server/src/cloud/bootService.ts:1179:26)
    at cli.service.reconcile (file://<checkout>/apps/server/src/cli/service.ts:225:31)
    at cli.service.reconcile (definition) (file://<checkout>/apps/server/src/cli/service.ts:88:40)
    at cli.service.run (file://<checkout>/apps/server/src/cli/service.ts:222:5)
    at cli.service.run (definition) (file://<checkout>/apps/server/src/cli/service.ts:200:34) {
  [cause]: BootServiceCommandError: Background setup failed while enabling lingering for this user (exit code 1).
      at file://<checkout>/apps/server/src/cloud/bootService.ts:1036:11
      at file://<checkout>/node_modules/.pnpm/effect@4.1.3/node_modules/effect/dist/internal/effect.js:1419:160
      at file://<checkout>/node_modules/.pnpm/effect@4.1.3/node_modules/effect/dist/internal/effect.js:2418:126
}

AFTER service.env:
CUSTOM="first
T3CODE_LAN=false
last"

Parsed with node:util.parseEnv:
{
  "CUSTOM": "first\nT3CODE_LAN=false\nlast"
}

CUSTOM preserved: false
Own T3CODE_LAN entry is false: false

The env rewrite precedes service-manager validation.
Installation then stops because isolated buses are unavailable.