docs(env): synth resource requirements

[hkimw]

Summary

• Add a Vivado synthesis resource policy for PR fix(rtl): resolve mdrv-1 multi-driver net for full-top synth #108 resource-terminated synth attempts.
• Document target host sizing: 64 GiB RAM target, 32 GiB exploratory minimum, 64 GiB swap, and PCCX_VIVADO_JOBS=1 default.
• Link the policy from timing evidence, Vivado README, docs index, and release evidence checklist.

Verification

• git diff --check

Notes

• Docs-only change.
• BLOCKED_RESOURCE_TERMINATED is documented as missing synthesis evidence; the PR body makes no synthesis, implementation, timing, or bitstream readiness claim.