feat: add WallTime and simplify Timestamp API

[algebraic-dev]

This PR adds a WallTime type representing a point in time as nanoseconds since 1970-01-01T00:00:00 local time. It also removes the sinceUNIXEpoch and AssumingUTC suffixes because Timestamp implies UTC, and WallTime implies it is based on the WallTime epoch (defined in the comment as 1970-01-01T00:00:00).

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase aa6fa1cf1a7c7a7161ceeda64c934176abc3595b --onto ea6e76707835317858b7dd36c95322679c50aaac. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-07 04:09:09)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase a416b90d22990f84607168ac6c520b52c843a25d --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-11 13:31:42)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase aa6fa1cf1a7c7a7161ceeda64c934176abc3595b --onto 3fc99eef102549c743c6c63547983319bfae6f01. You can force reference manual CI using the force-manual-ci label. (2026-05-07 04:09:11)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase aa6fa1cf1a7c7a7161ceeda64c934176abc3595b --onto 5d5642107d0433519265f155ddbfbfb98007a80b. You can force reference manual CI using the force-manual-ci label. (2026-05-07 18:20:37)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase a416b90d22990f84607168ac6c520b52c843a25d --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force reference manual CI using the force-manual-ci label. (2026-05-11 13:31:44)

[TwoFX]

Looks good!