Trampoline Feature for Hardware Tasks

This pull request introduces a new trampoline feature for hardware tasks in RTIC.
It allows an interrupt handler to pend another interrupt (the trampoline), which then executes the actual task code.
This approach improves interrupt handling flexibility and source masking safety (as some exceptions cannot be masked through the NVIC).

The implementation includes changes to parsing, code generation, and error checking for correct usage.
It also includes a small CI fix and a minor clippy compliance improvement to the rtic-monotonics crate.

Overview

Trampoline Behavior

Normally, when using the source masking backend, we expect that a lower-priority exception waits until the higher-priority task finishes:

  lock     |     _______________
           |     |             |
  exception|     |             | =========
  task     | ____| _  _  _  _  |_
           |_______________________________

However, because we artificially raise priority by disabling interrupts (not exceptions), the effective task priority remains unchanged.
This means that lower-priority exceptions can still preempt the task:

  lock     |     ______          ______
           |     |                    |
  exception|     |     ==========     | 
  task     | ____| _  _          _  _ |_
           |_______________________________

The trampoline mechanism solves this by pended interrupts that are properly masked by the source masking backend, preserving the Stack Resource Policy (SRP):

  lock     |     ______      ______
           |     |                |
  exception|     |     =pend=     | ~~trampoline irq~~
  task     | ____| _  _      _  _ |_
           |___________________________________________

As illustrated, there is a brief priority inversion when pended, but since no critical section is executed during this window, it is not problematic.
We also do not enforce a trampoline when shared resources are not used — these cases still involve a minor priority inversion, but since shared data is untouched, this trade-off was accepted.

For more discussion, see issue #1088.

Implementation Details

• Added support for the trampoline argument in hardware task definitions, allowing an interrupt handler to pend another interrupt that executes the task logic.
  [1]
  [2]
• Updated code generation: when a trampoline is specified, the original interrupt handler only pends the trampoline, which then runs the user’s task code.
  [1]
  [2]
• Improved source masking logic to account for trampolines and added checks preventing illegal combinations of exceptions, trampolines, and shared resources.
  [1]
  [2]
  [3]

Documentation and Changelog Updates

• Updated changelogs to include the trampoline feature and its interaction with source masking.
  [1]
  [2]

Miscellaneous Improvements

• Changed modulo operations to is_multiple_of() in rtic-monotonics for clippy compliance.
  [1]
  [2]
• Fixed a CI issue caused by a leading space in a test output file.

Summary

These changes collectively enhance the flexibility and safety of interrupt handling in RTIC while improving CI reliability and code linting compliance.